Magic Vault Defense

[Tenmujiin]

So I have reason to believe (one of them told me) that my players will try to steal from a heavily guarded vault with a large number of "elven blades" (daggers, short swords and longswords that deal 1d4 to 3d4 bonus damage of varying types) as well as a number of other magic items, basically 30% of the magic items in the campaign area are contained in this vault, still only about 20 items other than the elven blades but still a serious amount of valuables.

The civilisation that built the vault is based on a highly magical version of the roman empire with permanent magical effects featuring heavily in their cities and almost every soldier having an enchanted weapon (specifically the elven blades) and the vault was built on the frontiers of their conquest. The vault is currently held by the remnants of that civilisation (it was destroyed by a magical cataclysm).

My question of you guys is this: what defenses would be installed in such a vault and what PC abilities should I watch out for? The only classes I know I'll be dealing with are fiend pact tomelock and changeling assassin, the rest of the party is either just joining in the next session or had their character die in the last one. All of the PCs will be level 5.

[Ninja_Prawn]

Well, let's see. A 5th-level warlock has access to Dispel Magic, right? That could potentially allow them to erase a couple of magical wards per short rest... They would most likely want to save their slots for big, easily-identified effects like Hallow and Guards and Wards.

So, you need to make sure there's a decision to make between using slots for dispelling and using them for other things. You could also hide discrete (and discreet) instances of Arcane Lock within an area protected by Guards and Wards, just as a fun surprise.

Glyph of Warding is an important tool because it is invisible. You have to be careful with invisible traps though, because they can feel like 'gotchas'. Try and leave hints about where they are what the passwords might be, or how to avoid them.

Don't forget illusions! They can be hard to use, but it's a nice way to add some variety to the defences. Also, animated objects or golems give your non-sneaky party members something to do.

[JellyPooga]

The first and foremost question I need to ask is; do you want the PC's to successfully raid this vault?

If you do, then you need to plan the defences accordingly; take note of character class and level, adjust for "appropriate encounters" and so forth.

If you don't want them getting in, but would enjoy seeing how they cope with your diabolical defences, then just go to town on it.

Here's some "layers" of defence to consider;

1)Guards. Cheap, disposable and an effective deterrent. Just having them be there, all nice and visible, will cause your players all kinds of strife.

2)Locks. A locked door is an obstacle. If the would-be-thieves are trying to stealth their way in, this should give the Guards more time to spot them.

3)More Guards. Did I mention that they're cheap AND disposable? Put some behind the locked door as well as outside it.

4)Another locked door. We don't want the Guards going places we don't want them to, do we?

5)A Labyrinth. Nothing says "go away" like getting lost in a maze. Make sure the entrance gives the thief a good look at the size and complexity of it from a good vantage point...but make the view is an illusion; you don't want some clever sausage mapping it out from said vantage and waltzing through.

6)Traps. Anyone persistent enough to get past a second locked door and has decided to brave the labyrinth deserves a trap to the face. They've had plenty of warning. Fill that maze with the suckers.
6a)Start Cheap, Finish with a bang.
- We don't want to waste money on pricey traps right by the door. Start with traps that don't cost much; pit traps (with spikes!), spear traps and so forth. Then work your way up to the better ones; falling blocks, rolling boulders, magical traps, etc.
6b)Don't forget they're tricksy so-and-so's. Trap the floor, trap the walls, trap the ceiling, trap the air, trap the water, trap the doors, trap the way forward AND the way back (or better yet, DON'T trap the way forward, THEN trap the way back). If you can fit a trap on it, trap it.

7)Guardians. These differ from Guards inasmuch as they tend not to be so cheap. Golems, Undead and other critters that don't need to be fed all make good Guardians. Bound Fiends are a particular favourite of mine; they tend to be a little peevish at being trapped in a little circle for however many years (hundreds? thousands?). You put these after the labyrinth because you don't want to have to replace them. If you can swing it, get them to maintain the labyrinth (and its traps) too.

8)The Door. No, not just a door, the Door. It's probably quite large, made of stone, ornately carved and with dozens of locks and traps. It's quite obviously the last door before the vault. It also doesn't open. Behind it is a blank stone wall, because...

9)Bait and Switch. Put the loot somewhere else (https://xkcd.com/916/). Make sure everyone knows that the loot is in the death-filled maze of doom. Occasionally let thieves get in and out again; there must be something valuable beyond the maze of doom if it's that deadly and guarded so well, right? Alternatively, charge entrance.

[Ninja_Prawn]

You could also go for some themed areas. Hallow can create permanently silent areas, and you can easily store a Silence spell in a Glyph of Warding or have a guard on hand to cast it. You could even have a mundane anechoic chamber that messes with your PCs' perception.

Maybe include some Ear Muffs of Thunder Resistance in the treasure, as a nice reminder of the adventure.

[Alejandro]

So I have reason to believe (one of them told me) that my players will try to steal from a heavily guarded vault with a large number of "elven blades" (daggers, short swords and longswords that deal 1d4 to 3d4 bonus damage of varying types) as well as a number of other magic items, basically 30% of the magic items in the campaign area are contained in this vault, still only about 20 items other than the elven blades but still a serious amount of valuables.

The civilisation that built the vault is based on a highly magical version of the roman empire with permanent magical effects featuring heavily in their cities and almost every soldier having an enchanted weapon (specifically the elven blades) and the vault was built on the frontiers of their conquest. The vault is currently held by the remnants of that civilisation (it was destroyed by a magical cataclysm).

My question of you guys is this: what defenses would be installed in such a vault and what PC abilities should I watch out for? The only classes I know I'll be dealing with are fiend pact tomelock and changeling assassin, the rest of the party is either just joining in the next session or had their character die in the last one. All of the PCs will be level 5.

As an alternate idea, why not have a powerful diviner assigned to the vault, whose task is to foresee any attempts to rob it? Assuming you are trying to prevent its being robbed by the PCs, the diviner has many ways to know what is about to happen ahead of time, and then they can take the simplest step: removing the 20 items plus the elven blades, and put them somewhere else. Then bust the PCs when they break in. :)

[Douche]

Keep a hydra or something in the vault. That's how I'd make sure even if they do get in, there'll be a bunch of angry gnashing teeth waiting on the other side.

But if it was my vault I'd probably also leave a hidden switch to release sleeping gas or something, so I could easily go in and get my valuables on demand.

[Slipperychicken]

My question of you guys is this: what defenses would be installed in such a vault
They would have alarm systems to signal response-teams. Of course, since those teams are long dead now, the alarms will probably just annoy the PCs and signal nearby opportunists that someone's trying to crack the vault.

Also, there's the question of why the vault hasn't been cracked yet. That may help you design it.

[Fwiffo86]

I am reminded of why old school lichs were never beatable.

Read this only if the creators of the vault had access to Wish at the time of planning and construction of the vault.

Day 1 - "I wish to know the exact date(s) my vault will be infiltrated.
Day 2 - "I wish to know the exact time on [date] my vault will be infiltrated.
Day 3 - "I wish to know the exact location my vault will be entered on [date/time].

If you have intelligent undead guardians, (see aforementioned lich)...

"Oh, its ten minutes until the vault is breached at x location. <<teleport to location>>. Begins casting spells - Gate - demon, gate - demon, gate-demon "ok boys, start summoning your own reinforcements", Delay blast fireball, Delay blast fireball, delay blast fireball, Lightning bolt contingency, Lightning bolt contingency, Ward, Ward, Ward....

Party enters location....

BOOM.

Again, this is old school.. Many of these spells no longer have equivalents.

[Fable Wright]

So what you have here is not so much a vault as a fortress. A heavily-guarded military garrison, given that it's supplying troops, and it's on or near the front lines. Let's think of what kind of defenses a military garrison might have, now shall we?

1. Guard towers around the compound manned by animated ballista. They cannot be bribed, cannot disobey orders, and cannot become tired. If you enter without authorization, they will fire on you. If you loiter in the vicinity, they will fire on you. If you come at them with heavy tanks, they will release the golems or use their innate spellcasting to cast Shatter on them.
2. Distance between the outer walls of the compound and the compound itself. It takes several rounds under heavy ballista fire to cross this, and every route but the main entrance is mined with traps. The main entrance merely has extremely tight security on the door, several checkpoints, and traps for those who decide to wander off it.
3. Alarm systems and guards. While the guards themselves have long passed, the magical catastrophe has caused many of the less-sentient denizens to grow drastically in size. All have the same instincts about the alarm: When it goes off, food fresher than MREs has entered the building. Giant spiders, Spectators, Nothics, whatever abberations you need. They live here.
4. Access to the weaponry is going to be restricted. Try to access a weapon that's not yours, and the weapon's going to target itself with Heat Metal. The doors will also lock, and spectral guardians may or may not arise to attack the intruder.

Feel free to change the layout if that doesn't fit what's in your head, but that's how I'd start planning the compound.

[Zman]

A vault where the creating magical empire has been destroyed through magical cataclysm....

Um that screams timeless order following guardians, ie Iron Golem. Maybe a couple of magical traps.

Now, if you want the PCs to be able to raid this vault you could drop hints about there being some kind of ritual that disables the guardians etc, even let them have a failing encounter with the Iron Golem, just make sure you provide them a way to survive ie run with their tail between their legs.

[CNagy]

I think the easiest way is to have permanent magic effects layered on objects in such a way that casting a Dispel magic is going to hit multiple spells. A variant of Unseen Servant, a Tenser's Floating Disk, whatever, is keeping a switch depressed, and accidental dispelling (like if you were trying to dispel actual magic defenses and didn't know that seemingly innocuous one was there) causes the switch to become unpressed, resulting in a many-tons stone gate to descend from the ceiling. And it has Glyphs and Symbols etched on its previously unseen surface.

Or the hallway is actually a deep pit bridged by a Wall of Wood (think variant Wall of Stone, permanent until dispelled rather than permanent period) upon which stone plates (some trapped) have been set to give the appearance of a stone hallway. Attempt to dispel magic defenses in that hallway and you may dispel the Wall acting as a floor, causing the entire hallway to become a dead-drop.

You could have permanent Teleportation Circles serving as doors closest to the vault and sharing space with the most dangerous magic defenses. Accidentally kill the Circle and you have no immediately visible way to progress. Mix in trapped Circles with necessary Circles; like stepping on a Circle in one place may teleport you to a Circle inscribed on the ceiling of another room, where you proceed to fall 30 feet into knee deep water hiding swarms of carnivorous fish and access to an underground river that leads you away from the vault.

Once the party has made things harder for themselves a few times by trying to take the easy way out with Dispel Magic, they'll start looking for more creative ways to get through the traps.

[Tenmujiin]

Thanks for the responses guys.

A few things I forgot to mention. The mundane soldiers ans some of the weakest casters from the empire have survived and their decendants are holding the city in which the vault is located. I has only been about 70 years since the cataclysim which is slightly under the lifespan of elves in my setting. The PCs are currently in the employ of the army and have been inside the vault to be gifted a magical item each as the task the were given was more than they could handle otherwise (they still lost half the party due to bad rolls but thats another story). I was highly non-specific about the vault as I didn't prepare for that session as throughly as I perhaps should have...

While I'm willing to have them successfully raid the vault I don't want it to be easy. I run a somewhat sandbox campaigh so if they decide to raid this place at level 5 after I drop hints it probably isn't a good idea then a few party members dieing is on them. At the same time I don't want it to be impossible and the vault serves as an armory and so needs to be accessed regually by the army.

[JellyPooga]

Extradimensional Space is your friend.

Use Mords Magnificent Mansion as the basis for your extradimensional vault; the spell specifies that only the caster and designated creatures can enter, that you get to create any floor plan and that you choose the furnishings. Design the floor plan and furnishings to bar access to the actual storage area for anyone except the near-invisible servants.

So, what we want is several Magic Items that allow you to access and use the vault;

1) An item that summons the door and allows the wielder to determine who may enter.
2) An item that allows you to command the invisible servants within. Without this, you cannot retrieve or store anything in the vault area.
3) An item that allows you to designate items as being allowed to leave or not (i.e. it can either put or remove a kind of magical bar-code on an item). A "coded" item cannot be taken out of the portal (it merely remains behind if you try to leave with it).

I'd make (1) and (3) wands or rods and (2) an amulet or ring. Without all three items, you can't get anything out of the vault.

Give each item to a different person. (1) could be held by the local ruler, (2) could be a badge of office for the local law-enforcement and (3) belongs to the Court Magician.

The PC's will have to discover how the vault operates and obtain all three items in order to loot the place. Given that the items are held by the powers that be, getting hold of each of them could be a mission in itself;
- The Rulers Palace is guarded by elite soldiers and excellent exterior fortifications. Inside there are many potential pitfalls and perils and the place is practically a maze; don't stumble in on the wrong room or you'll bring the entire guard down on your head...
- the Court Magicians mansion is festooned with magical traps, summoned creatures and animated objects.
- The Local Sheriff sleeps in the guard house, which is guarded 24/7. Did I mention that the Sheriff use to be an adventurer?

[Tenmujiin]

Extradimensional Space is your friend.

Use Mords Magnificent Mansion as the basis for your extradimensional vault; the spell specifies that only the caster and designated creatures can enter, that you get to create any floor plan and that you choose the furnishings. Design the floor plan and furnishings to bar access to the actual storage area for anyone except the near-invisible servants.

So, what we want is several Magic Items that allow you to access and use the vault;

1) An item that summons the door and allows the wielder to determine who may enter.
2) An item that allows you to command the invisible servants within. Without this, you cannot retrieve or store anything in the vault area.
3) An item that allows you to designate items as being allowed to leave or not (i.e. it can either put or remove a kind of magical bar-code on an item). A "coded" item cannot be taken out of the portal (it merely remains behind if you try to leave with it).

I'd make (1) and (3) wands or rods and (2) an amulet or ring. Without all three items, you can't get anything out of the vault.

Give each item to a different person. (1) could be held by the local ruler, (2) could be a badge of office for the local law-enforcement and (3) belongs to the Court Magician.

The PC's will have to discover how the vault operates and obtain all three items in order to loot the place. Given that the items are held by the powers that be, getting hold of each of them could be a mission in itself;
- The Rulers Palace is guarded by elite soldiers and excellent exterior fortifications. Inside there are many potential pitfalls and perils and the place is practically a maze; don't stumble in on the wrong room or you'll bring the entire guard down on your head...
- the Court Magicians mansion is festooned with magical traps, summoned creatures and animated objects.
- The Local Sheriff sleeps in the guard house, which is guarded 24/7. Did I mention that the Sheriff use to be an adventurer?

While that is a great idea. The PCs have already been inside the storage area, I just never described it more than vaguely. Them seeing the number of "magic swords" was what prompted them wanting to steal from it.

Edit: While I can basically design this to be whatever is needed, the storage area needs to be fairly easily accessed if you are allowed in.

[Kane0]

So make it hard to get that permission. Modify all other variables you have not already set.

Need paperwork signed by at least three ranking officers unless you're escorted in. Make sure that people masquerading as officials can be discerned fairly accurately (both via regular resguises as well as charms/illusions/polymorphing).

Have the outer areas secured by military personnel and their equipment, ready for anything. This includes watches and contingency plans if they are infiltrated or attacked. Trespassers will be assumed hostile.

Have the inner areas prior to the vault itself have easily but temporarily deactivated traps, including lockdown procedures. Those not allowed are targetted indiscriminately.

Have the swords tagged with alarms or other magic. If taken without being deactivated, the consequences could range from an alert to a self destruct or sigil trap, or from being locked in place or teleported to an emergency location. The greater the magic item, the more wards such as these are in place.

[JellyPooga]

Them seeing the number of "magic swords" was what prompted them wanting to steal from it.

Heh. This is why you don't let murder-hobos into your vault to see your vast wealth!

You say the PC's are level 5, right? That's not so bad-ass that they shouldn't fear the local authorities. Sand-box or no, you really need to impress upon them the foolishness of attempting this; if these people have an army equipped with magical gear, I'd be very worried about what they have protecting their more valuable stuff.

If they still want to go ahead with it, you need to pull out all the stops. For example;
- Have the place guarded by a small army; not just a couple of guards, whole squads patrol the grounds surrounding it. They patrol with dogs...really big dogs. Or Blink Dogs. Or really big Blink Dogs.
- Every item of any value is protected with a Glyph of Warding (Spell Glyph): Dimension Door, triggered on touch, programmed to teleport the target up to 500ft directly into a prison cell (or place of choice; dungeon, slime pit, death trap, Owlbear nest...anywhere you like really).

[Tenmujiin]

Heh. This is why you don't let murder-hobos into your vault to see your vast wealth!

You say the PC's are level 5, right? That's not so bad-ass that they shouldn't fear the local authorities. Sand-box or no, you really need to impress upon them the foolishness of attempting this; if these people have an army equipped with magical gear, I'd be very worried about what they have protecting their more valuable stuff.

Well the reason the PCs were allowed in was because the army is short staffed dealing with an invasion. The weapons are only slightly more powerful than normal weapons, most of them deal an additional 1d4 damage of varying types (elemental or slashing).

I have outright said that this may not be a good idea but the warlock seems confident that he can pull it off if I've left any weaknesses. He has specifically asked for a detailed description of the parts of the vault the party saw so I'm definitely worried :smalleek:

My main ideas so far (other than wards/traps) are:

What they saw was an illusion, the vault looks quite different when not "active".
Guard animals/constructs. The constructs in particular would have plenty of reason not to be on the front lines.


I should also mention, the magical cataclysm mostly affected the homeland, the area the players are in was only effected in the areas around some of the local mage towers. Also the weapons are 'locked' so that they do a grand total of 1 damage (no ability modifiers) and can only be unlocked by the correct ritual or a high DC arcana check. The other magical items have similar locks that prevent the item's magical effects from being used. This was established in a previous session and the army does not have the ritual to unlock them (all of the mages with knowledge of more than combat spells were in the homeland at the time of the cataclysm and can be assumed dead). The spells in the PHB and various splat are the extent of modern magical knowledge but since the vault and magical items were constructed before hand they can have effects not listed).

Sorry I keep coming back with new information, I keep terrible notes so most of this is stored in my head.

[JackPhoenix]

Don't forget you don't have to have a security balanced for a party level. If, when the lost empire built the vault, the threats they were building it against were around level 15, the party is over their head if they decide to break in at level 5...and they should know it, or have a way to learn that the vault was built against thieves much more skilled and equiped.

[JellyPooga]

He has specifically asked for a detailed description of the parts of the vault the party saw so I'm definitely worried :smalleek:

This is the easiest part to deal with. Make what he's seen as simple as possible; a door, a corridor, a second door and the vault. Simple looking doors, simple looking corridor, simple looking vault. Nothing untoward, just well crafted stone walls. No windows, no grates; one way in and one way out.

This is the "easy" way in, usable by those with the correct pass-phrase, item-key or what-have-you at the front door (this would be besides getting past any guards, whatever that requires).

If you break in without the appropriate "key" (whatever form it takes), the front door takes you someplace else; make the "someplace else" still access the vault as a kind of emergency back-up entrance in case the key is lost, but make it all kinds of deadly and/or misleading in there.

[Vogonjeltz]

So I have reason to believe (one of them told me) that my players will try to steal from a heavily guarded vault with a large number of "elven blades" (daggers, short swords and longswords that deal 1d4 to 3d4 bonus damage of varying types) as well as a number of other magic items, basically 30% of the magic items in the campaign area are contained in this vault, still only about 20 items other than the elven blades but still a serious amount of valuables.

The civilisation that built the vault is based on a highly magical version of the roman empire with permanent magical effects featuring heavily in their cities and almost every soldier having an enchanted weapon (specifically the elven blades) and the vault was built on the frontiers of their conquest. The vault is currently held by the remnants of that civilisation (it was destroyed by a magical cataclysm).

My question of you guys is this: what defenses would be installed in such a vault and what PC abilities should I watch out for? The only classes I know I'll be dealing with are fiend pact tomelock and changeling assassin, the rest of the party is either just joining in the next session or had their character die in the last one. All of the PCs will be level 5.

So...they want to break into and loot what is functionally a military armory?

Presumably the guards are actually using these weapons, and there are probably a fair number of them plus a warning system to summon 'many' more in the event of an overt attack. Given the inherent value of what is contained within, it would be shocking if every guard wasn't already using these weapons as well as it would be ludicrous to lock up the best weapons and use butter knives or whatever instead. So, if the guards aren't using these weapons (or better) there would need to be a really good explanation, especially in a medieval society where weapons can make or break it.

I would assume there are permanent alarm spells and arcane locks in addition to physical locks and bars. There does reach a point where additional defenses (especially ones that would have been costly to cast) become extremely unlikely, especially if the weapons contained therein are, relative to the civilization that set up the magical defenses (if any) basically commonplace.

You don't build a vault for the everyday dinnerware, you probably don't even lock it up. The fine silver, on the other hand, might warrant it.

Again, if these are really just normal weapons for the civilization, it would probably just have some locks and a few guards, because they weren't special enough when the place was constructed to make it worth adding any other defense.

[Tenmujiin]

So...they want to break into and loot what is functionally a military armory?

Presumably the guards are actually using these weapons, and there are probably a fair number of them plus a warning system to summon 'many' more in the event of an overt attack. Given the inherent value of what is contained within, it would be shocking if every guard wasn't already using these weapons as well as it would be ludicrous to lock up the best weapons and use butter knives or whatever instead. So, if the guards aren't using these weapons (or better) there would need to be a really good explanation, especially in a medieval society where weapons can make or break it.

I would assume there are permanent alarm spells and arcane locks in addition to physical locks and bars. There does reach a point where additional defenses (especially ones that would have been costly to cast) become extremely unlikely, especially if the weapons contained therein are, relative to the civilization that set up the magical defenses (if any) basically commonplace.

You don't build a vault for the everyday dinnerware, you probably don't even lock it up. The fine silver, on the other hand, might warrant it.

Again, if these are really just normal weapons for the civilization, it would probably just have some locks and a few guards, because they weren't special enough when the place was constructed to make it worth adding any other defense.

The weapons WERE standard issue and part of the reason these guys conqured a good portion of the known world but it's been almost a generation since knowledge of their construction was lost (along with just about everyone capable of reverse engineering them).

The vault's original purpose was to store particually valuable items when they were not in use and it's location in a frontier city means it would have been built to be able to deal with hostile adventurers.

[tieren]

I have outright said that this may not be a good idea but the warlock seems confident that he can pull it off if I've left any weaknesses. He has specifically asked for a detailed description of the parts of the vault the party saw so I'm definitely worried :smalleek:



The details you provide could help set the trap too though.

For instance what if in giving the detailed description you tell him he noticed, almost unconsciously a "wrongness" about the space or a shimmering of the walls of the vault other than where the weapons they were given were stored.

Then if they break back in they can find many of the stores to be illusions to impress visitors and mislead them to the strength of the garrison, or mimics or some other trap.

Tipping them off in the description may help deter them from that path.

Another thought I had would be making one of the antechambers before the vault itself actually a permanent teleportation circle to an identical antechamber at a destination far away (perhaps the original capital city of the empire). Someone with the correct item or key enters the antechamber at the frontier and exits the one at the capitol to access the secure vault. Anyone else either needs to cast teleportation themselves (good luck at 5th level) or opens the far door to access a broom closet.

[MeowfaceMcGee]

Every army has a quartermaster. If you want an easy way in then you need the quartermaster or something from his possession.

Have you ever tried taking something off of one of those guys? You're party better be really good at killing important military personnel, extremely super good at convincing people or very very sneaky.

Other than that just fill it to the brim with traps, mazes/misdirections, guards, automated defense systems, you name it. Make it basically (but not quite) impossible to enter without the quartermaster's say so.

[Slipperychicken]

I wonder if Leomund's Tiny Hut could be used for access-denial. Make such an area (a hallway perhaps?) that the spell covers the entire passageway. You get nine creatures who are allowed to access the most sensitive parts of the vault, have them in the spell when it goes off. Have spellcasters (or any 5th level character with Ritual Caster) come in to re-cast the spell every 6-7 hours, so it doesn't expire.

Now you have a barrier of force through which only ten creatures may pass (nine plus the caster). Anyone else trying to break in needs to deal with it somehow. And one or two of the ten could be sitting inside or behind the hut, ready to send for help if they notice anything suspicious. And then you can incorporate such a barrier into a conventional security system. Perhaps it could be behind a checkpoint at the entrance to the vault.

[Vogonjeltz]

Every army has a quartermaster. If you want an easy way in then you need the quartermaster or something from his possession.

Have you ever tried taking something off of one of those guys? You're party better be really good at killing important military personnel, extremely super good at convincing people or very very sneaky.

Other than that just fill it to the brim with traps, mazes/misdirections, guards, automated defense systems, you name it. Make it basically (but not quite) impossible to enter without the quartermaster's say so.

It can't be too crazy, the people who are supposed to be there would need to actually be able to get the arms in a time of crisis.

IMO, make it like a bank, a giant vault door with an Arcane Lock and an Alarm spell. Anyone who was meant to open it (quartermaster) would know the password, and anyone who wasn't would probably trigger the Alarm alerting all the guards in a wide area.

This way the players have to either get someone to open the door and disarm the alarm, or steal all of that (sneakers style), or try and just bust in and grab as much as they can carry before making a mad dash for the exit.

It could be exciting as a heist regardless of the method chosen.

[JackPhoenix]

The weapons WERE standard issue and part of the reason these guys conqured a good portion of the known world but it's been almost a generation since knowledge of their construction was lost (along with just about everyone capable of reverse engineering them).

The vault's original purpose was to store particually valuable items when they were not in use and it's location in a frontier city means it would have been built to be able to deal with hostile adventurers.

If they were standard issue, why is only few of them left? Generation away isn't that much, if there were thousands of soldiers with magic weapons acting as an army. What happened to their blades? Also, if they were standard issue, they wouldn't have been so well protected back when the empire was at its height, so any defenses to protect them would've been created after the fall, with much diminished "technology level".

[Temperjoke]

Okay, so they're basing their theft on the parts that they saw? Perfect, that means that they can be caught off-guard by what they didn't see: The doorway they based through was a magical portal that takes them directly to the storage area of the vault, past 90% of the traps, but they won't be able to use that without key to unlock the door in a certain way. Think the keys/doors in Matrix Reloaded, one set opens like normal in the Matrix, while if the tumblers are set in a different combination, the door leads elsewhere.

And that "elsewhere", in this case, is a storage room beneath several layers of security. Now, something to consider is what would the traps be designed to do? See, when you design your security, you don't necessarily want them to kill the intruder, at least not the first or second layer. Someone caught in those traps could have been an accident, like a janitor who bumped the wrong button or stepped in the wrong spot accidentally. You also don't want to kill someone when you could imprison them for interrogation or use them as a public example of what happens to intruders. You also don't want to cause the entire base to go on alert for what could be a simple accident. So here would be my strategy:

Outside Door: Can let them straight into the vault if they think to get the key from the person who let them in the last time they were here, like the quartermaster or whoever the NPC was that rewarded them.

Layer 1: Simple trapped floor with silent alarms that alert the guard station, basic security doors with normal locks, containment walls that slam into place if tripped; more mundane security intended to stop further penetration by amateurs or people accidentally getting in.

Layer 2: More magical protections such as wards, runes, alarm spells. Fake rooms that have a darkness spell combined with a glue trap coating, or fake doors that set off alarms when you attempt to open them. Containment/Capture is still a focus here, but with more potential for lethal results, as it is highly unlikely that someone would accidentally get in to this layer.

Layer 3: At this point, stopping the intruder is a higher priority than capture. Spike traps, both in the floor or dropped from the ceiling, flamethrowers in the walls, rooms that fill with water, explosive traps, etc. There are also alarms that alert the entire base, causing troops to mobilize and make a perimeter, in hopes of stopping intruders from leaving.

(I should mention that the intent is that with each level, you are adding that layer's stuff in addition to the previous layer. So Level 3 has security layers 1, 2, and 3 on it; it might catch them off-guard if they forget to check for mundane security while dealing with magical)

Layer 4: Add in security monsters, things that can go without food or water (not like you're going to feed them with anything other than intruders) such as skeletons, living statues, living suits of armor, etc.

Vault room: So if they've made it this far, inside the vault room itself they can be attacked by a group of invisible assailants who are using the weapons that the thieves were attempting to steal! Normally impassive when someone uses the key to enter the vault room, they use the room's contents against anyone who enters, and, if something isn't done, the last one alive attempts to trigger a massive explosive trap that destroys the entire storage building, since it's better to destroy it all rather than let the enemy have the arsenal.