Unbeatable Castle

[Jaxter Gronaldi]

So, a friend of mine needs help coming up with a castle that cannot be penetrated, magically, or physically, and so I need ideas for some spell reccomendations.

[Jacquerel]

In 5e, with almost no access to permanent enchantments, there isn't any way within the rules for a player to build a fortification that is completely impervious or even one that is mildly resistant to magic. Especially high tier magic.
We can come up with things for you but they'd be entirely in the realm of "here's a cool plot device" rather than "here's something that exists within the game rules".

[qube]

Best thing I can come up with is a demiplane with a forbiddance.

Then again, there will be no 100% proof castle (if nothing else: you will probbably wanna use it too; at which point, top of my head: dominate person (or crit persuasion/intimidation check) to force you to let me in )

[Jaxter Gronaldi]

I meant for a high level npc, so can use any spells, just don't want players to smash or remote bomb because that's where the macguffin plot clue is.

[Jacquerel]

The point is that 5e doesn't really have support for basically any kind of perpetual enchantment, whether from PC or NPC. You could write a perpetual version of any existing spell but it would be homebrewing. Which is fine, but means that it's difficult to make suggestions.
It'd also help to know what the players are capable of. Some kinds of magic (brute force) are easier to defend against than others (teleportation, wish).

Obviously the easiest way to prevent magic would be to place a permanent anti-magic zone across the fortress and also several miles of the surrounding area, but that comes with its own problems and still isn't impossible to get around.

[KorvinStarmast]

The point is that 5e doesn't really have support for basically any kind of perpetual enchantment, whether from PC or NPC. You could write a perpetual version of any existing spell but it would be homebrewing. Which is fine, but means that it's difficult to make suggestions.
It'd also help to know what the players are capable of. Some kinds of magic (brute force) are easier to defend against than others (teleportation, wish).

Obviously the easiest way to prevent magic would be to place a permanent anti-magic zone across the fortress and also several miles of the surrounding area, but that comes with its own problems and still isn't impossible to get around.
There is the arduous process of making Teleportation circles permanent. :smallbiggrin:
True Polymorph lasts forever (*until dispelled*) if you concentrate on it for the full hour, so it could be permanent but once someone discovered that there's something magical about the creature/object, the chance to dispel arises.

[tieren]

I meant for a high level npc, so can use any spells, just don't want players to smash or remote bomb because that's where the macguffin plot clue is.

If you are the DM just say it is so. You don't have to make it anything the players have access to or understand.

[smcmike]

There is no reason to search through the rules for a way to build such a castle. That's the wrong way to go about it. Just make it however you want, preferably with interesting vulnerabilities and good fluff to explain them.

[No brains]

Maybe you could build a stupid number of golems and live in the spaces between them? They aren't actually structures, so siege monsters can't deal extra damage to them and non-magical siege weapons would bounce off. If one section of golem gets too damaged, it can retreat into the cluster to get healed, possibly swapping out for a golem better suited to handling what's hurting them.

Also: make it a castle for tiny creatures. You won't need to worry about larger creatures breaking in if they can't fit.

Anybody know a good way to keep out gaseous or incorporeal creatures? Shadow Demons, Yochlols, oozes, and ghosts/shadows/spectres/banshees/wraiths/whatever can still get into a tiny-tunneled golem fort.

[JellyPooga]

I'm reminded of a certain Discworld wizard that locked himself in a box, protected by powerful wards and enchantments, to escape Death...shame he forgot the airholes....

[Unoriginal]

Go deep in the earth and create a pocket that's too far from even the most powerful Teleportation spell to reach- except a 3x3 space that is in range of such a spell, but only if you know a precise location to stand. Then create another pocket that can only be reached if you stand at a certain place in the first pocket. Cover the inside of both pocket with metal, and you should have a good start for a safe location.

[Jaxter Gronaldi]

Thanks a lot all of you÷

[JackOfAllBuilds]

If it is in the Forgotten Realms, Mythals are a relic of high magic that exsists. Can be as small as a fort or as big as a city. They can block certain magic while empowering others, have built-in exceptions (high elfs casting with a wand of a specific wood). Basically the Mythal can do whatever you want it to.

I'd also look at Mordenkainen's Private Sanctum from the Wizard list, and Forbiddance from the Cleric list. They can block teleportation/planar travel and scrying, then you just have to worry about flight/passwall/stoneshape/move earth. So a Forbiddance plus an anti-transmutation Mythal

[Kane0]

Priority 1: Stay hidden. Never being attacked in the first place is far preferable to having the best defense.
Priority 2: Attrition. Build your defenses not on the premise of indestructibility, but of redundancy. Don't pull a Death Star.
Priority 3: Self-sufficiency. No point having a perfect defense if you can be seiged out of it.

So based on this lets start with something simple:
Start with Tucker's Kobolds.
Situate the warren on its own demiplane, an astral pocket or similar location. The more remote and inhospitable the better, and if you can twist the laws of the realm to your advantage do so.
Reinforce all surfaces with whatever materials you can. Anything that impedes divination, conjuration or magic is general is extra handy.
Double down on tricks and traps to keep intruders on their toes. Unmanned, automatically resetting ones are the best.
Replace all kobolds with miniature golems and similar nasties. The most effective creatures are intelligent, loyal and do not sleep nor age.

You basically want to make it not worth the effort to find and attack you, even if one would want or need to. Make every enemy victory phyrric and every move costly while preserving all your capabilities and advantages. Even a basic 'iron golems in a dungeon buried into the side of a volcano' situation achieves this.

[Corsair14]

I remember in second edition reading about a castle that took the fact it was a fantasy world seriously. There the PCs grouped up to build their castle(because in 2nd you attracted large numbers of troops at certain levels) and went to the elemental plane of earth and recruited a tribe of stone giants(maybe owed them a favor, I don't remember) and then carved out a perfect dome of a mountain that had anti-magic properties. They then managed to carve out the inside of the dome and left a single entry tunnel killing fields. What they were left with was an almost impenetrable fortress that flyers couldn't simply just land in, magic couldn't port you in or let you cast on it, you couldn't tunnel under it and come up from under, covered with small arrow or siege weapon ports. Not sure how they did the carving.