How To Do Siege Battles?

[Scorponok]

Hello Giants,

In the future, my party may get popular enough to start commanding armies. 100 soldiers vs. 100 soldiers is easy enough to predict. If both are armed similarly, just do a 1 vs 1 fight a couple of times then multiply the results.

However, what if the PCs send 100 sword and shield soldiers to siege a castle filled with say, 50 crossbowmen? Is there any way to simulate taking a castle? I know there are a lot of variables involved such as how high the castle walls are, how much open space between the castle and the forest that provides cover from arrows, what each side is armed/armored with, and the skill layout of the typical soldier (like, how good are they at climbing walls) but I was wondering is there a quick and dirty way to determine who would win?

This of course barring PC involvement - I'd build an actual castle wall and play it out if they decided they wanted to be part of the siege. This is happening entirely off-table, and the PCs only hear about it when the mission is successful or not.

[Gruftzwerg]

Dunno if there are real rules for this, but the groups I played so far tend to use one of these 2 options combined with a bit regular combat for the PCs.

option A:
try to apply swarm rules. Either with only 2 big swarms fighting each other, or if you want more consistence, several smaller packs/swarms.
Once one side has lost (all dead) it's just up to you to determine how many have died on the winner side, depending on the dmg they have taken

option B:
calculate the % chance for one army/pack to hit another army/pack and multiply it with the average dmg of the used weapon/attack. Instead of rolling to hit and dmg rolls, just designate the calculated dmg to a target enemy and use either regular initiative or pack several together to a unit.

[Scorponok]

Another thing I haven't seen in the D&D books: arrow ranges at different heights. The bowman on top of the cliff will be able to hit the one on the bottom sooner than the bottom can hit the top. But by how much??

[Strigon]

Unless your players do something innovative, all that matters is that you give them consistent numbers. Work out a formula, and use it.

Give your players a chance to make some plans. Let them give you input. For example, can they starve the defenders out, or do they want to assault? What kind of losses are acceptable? What resources do the attackers have that could help them in a siege? Who's leading them? From this information, you can get a pretty good idea of what the battle would look like.

The important thing isn't that you give them a correct answer, it's that you give them a consistent answer. Chances are, your players won't know what it's like to take a medieval castle, especially not when magic's involved, so all that matters is that you give them an answer they believe, and they can work with. If it sounds reasonable to you that the attackers win, say they won. Maybe do some rolling, see how many survived, and inform your players of the outcome.

There's absolutely no need to simulate the entire siege if the players aren't there. It's a colossal waste of time.

[zlefin]

well, oyu could use the pathfinder mass battle rules to approximate things somewhat.

[Random Sanity]

Don't forget you need a significant force-strength advantage to attack a fortified enemy. To reliably capture that fort with 50 crossbowmen, you'd need anywhere from 150 to 500 swordsmen depending on how well trained and equipped they are. That's largely so they can absorb the beating they're going to take as they charge in and go about breaching/overcoming the fortifications while still having the fighting strength to get the job done quickly once they get to grips with the defenders. As a general rule of thumb, given roughly equal skill and gear, the attacker needs a troop advantage of between 3:1 and 5:1. (There's a bazillion potential modifiers to this - leadership, experience, morale, you name it - but that's the conventional wisdom.)


This is, of course, assuming that the attackers brought along a ram or some ladders or something to get them past the wall in the first place. You're not going to hand-scale a vertical stone-and-mortar wall if the builders were the least bit competent, full stop. For the sake of abstracting a horde of minions going off to grab a nice new outpost for their PC boss, you can generally work with the players to sort out a general plan to take the fort, which can be as simple as "knock down a good-size tree and make a battering ram out of it" or as elaborate as spending a couple weeks building fancy siege hardware.


If the attack force meets the force-ratio requirements and is equipped for the job, it's just a matter of making some dice rolls to simulate casualties, and Bob's yer uncle.

[Scorponok]

Don't forget you need a significant force-strength advantage to attack a fortified enemy. To reliably capture that fort with 50 crossbowmen, you'd need anywhere from 150 to 500 swordsmen depending on how well trained and equipped they are. That's largely so they can absorb the beating they're going to take as they charge in and go about breaching/overcoming the fortifications while still having the fighting strength to get the job done quickly once they get to grips with the defenders. As a general rule of thumb, given roughly equal skill and gear, the attacker needs a troop advantage of between 3:1 and 5:1. (There's a bazillion potential modifiers to this - leadership, experience, morale, you name it - but that's the conventional wisdom..

That's interesting. I have never heard of the 3:1 or 5:1 ratio to take a castle but it makes sense. Is there any historical precedence for this number?

[Random Sanity]

That's interesting. I have never heard of the 3:1 or 5:1 ratio to take a castle but it makes sense. Is there any historical precedence for this number?

It's more general military wisdom; when you look at famous castle battles, all the successful assaults involved an attacking army that dwarfed the defending force. Attackers that didn't have a major numbers advantage (or couldn't afford the losses involved) generally had to pen the defenders up and starve them out, which could take months.

The thing is, defensive structures are built in the first place because they're a major force multiplier for whoever's trying to control the area. You can easily fend off an army of a thousand soldiers with less than half that if your castle is well-built. Or if your trenches are well-dug, to carry the example forward to the 20th century. Cracking a fortified position by force is something no commander wants to do if there's an alternative, because his troops are going to take a hellish pounding doing it.


If you have the chance, the documentary series "Battle Castle" describes several famous castles and how they were taken (or not) by invaders. (At least one guy has it up on Youtube at the moment.)

[Anxe]

The 3:1 and 10:1 numbers come from Sun Tzu I believe.

There's a few supplements put out by Green Ronin that covered how to do large scale battles in D&D. The Trojan War or the Biblical one. I run a Classical inspired campaign, so I use the Trojan War rules. For the small scale conflicts you described they work great. It gets bogged down when you get into the thousands though.

There are rules for spells, morale, special combat maneuvers, and even a set of simple rules for single characters to take on a while unit at once. Everything is pretty easy to design and use.

That said, even though I have this supplement I more often have battle outcomes dictated by the story. Usually the PCs will engage in combat with an A-team from the opposing side and then whoever wins that conflict also wins the larger battle.

In the past I've also been tempted to use a video game system to simulate battles in D&D. So those 150 swordsman are attacking 50 crossbowmen? Design an Age of Empires 2 (or game of choice) scenario where one side has 150 longswordsmen and the other side has 50 crossbowmen behind a defensive structure. Who wins? Well there you go, that's the winning side.
The issue with video game battles is that there often isn't a morale system in place. Armies typically break and run after suffering 10-20% casualties. In my Age of Empires 2 example, the longswordsmen will fight to the last man as will the crossbowmen. Consider that if you decide to go for a video game simulation of the battle.

EDIT: For arrow ranges, there aren't official rules. Defenders will often have cover though, which gives them a +4 to AC. That's effectively two range increments. You could give them a longer range, but its unlikely that they will maintain accuracy at that range. What I've done in those circumstances is given a group of archers an area volley attack that requires a Ref save to halve the damage that one arrow would do.
These are all more complicated solutions than the quick and dirty you asked for but the truth is that a quick and dirty method is basically, "Which side would it be more fun to have win? Do the PCs need acknowledgement of their accomplishments right now or a new obstacle?" That's all you really need. Justify the desired outcome afterwards. Weather tends to play a tremendous role in how battles play out, so maybe it rained and the outcome the PCs predicted is spoiled by that.

[TheYell]

That's interesting. I have never heard of the 3:1 or 5:1 ratio to take a castle but it makes sense. Is there any historical precedence for this number?

I've heard it ascribed to Erwin Rommel in WWI fighting in Italy. 3:1 suppresses defensive firepower enough to allow the assault troops to move in relatively unopposed.

You might approximate it by letting a fortified unit fire twice and move not at all. Abandoning or moving into the fortification consumes a full move turn.