Well, they've done it.
My PCs have found themselves in a bind where an approaching army is set to invade while they're locked behind the walls of a small town and keep. Acting as spies I had hoped that they'd be in and out before the army arrived, but oh no, they had to go and have "Lord of the Rings" delusions of grandeur and now hope to hold off the approaching army of 2,500 soldiers with nothing more than themselves and the 350 able-bodied warriors the keep calls home.
I hate to say it, but I'm pretty sure they're screwed. However, I'm one to give all parties a chance, after all they surprised by getting into this mess, hopefully they'll surprise me by actually living through this.
Are there any rules for mass combat? One army taking on another?
I use a version of the Star Wars Saga Edition rules for mass battles. In essence, you convert groups into units, basically something similar to a swarm template, and each hit point represents a soldier. PCs can commando units and grant bonuses, and so on. Pretty smooth running.
I'd imagine the Kingmaker AP would have some specific to PF.
There are a couple sets of rules compatible with Pathfinder. These include: WARPATH—Rules for Mass Combat and Book of the River Nations: Mass Combat (containing rules created by Paizo for the Kingmaker AP), both available on Paizo.com.
I haven't had much experience with actually using either, but I like Warpath more.
You could always make your home team fight to the death (it's their home, after all), and the individual invaders flee at 75% or 50% health, with only the sergeants and above staying until 25% or death. Or if a leader of a squad is killed or knocked unconscious, the whole team flees.
Dont forget siege weapons on either side. Moving armies might have some, but they're hard to move and will slow the army down. The town might have much more. Use weather. Armies don't like to fight in the winter. Rain can make hazardous terrain. The town is better protected in harsh weather at night, the army in their tents might freeze.
Use mass poisons against the army. Spoil their food, steal their alcohol, have the rogue sneak in and spread around poisoned drinks among the men, giving them penalties fighting the next day.
Given that there is a small town next to the keep, I'd suggest arming the able-bodied members of the citizenry and having them help fend off the enemy army. That might help even the odds a bit.
Also I'd suggest having the PCs go straight for the enemy commander. It will likely cause the rest of the army to break and run.
That should be fun to DM. :p
I don't know of any published rules for something like this, but I can share with you what I would do.
1.) Either stat up your own Warriors and Adempts or pick some out from the lists here. Make them each 5-6 CRs below your APL. Use their stats for the incoming army.
2.) Vary the army by having some of the warriors act as archers, some use spears, some sword and shield, etc. Portion out the army however you like... I would probably do 1500 melee warriors, 500 archers, and 500 adepts. I would also replace a couple of the adepts with wizards and at least 10 of the melee warriors for army generals (fighters) who are 2 or 3 CRs above the others.
3.) Determine if the 350 "able-bodied" allies fighting with your party would be weaker, stronger, or about the same as the incoming army. If you think they're roughly equal, you can just use the same warrior/adept stats for them. Also make a few of those guys stronger than the rest, just like we did for the incoming army.
4.) When the battle ensues, we all know that it starts out with archery and catapults. I would roll a D100 5 times with a 15% chance each time of a catapulted boulders coming near the PCs. If nothing, then the boulders hit somewhere else in town. If one comes near, REFL DC 10 to get out of the way or take 4d6 and fall prone. If they're in a position to get hit by arrows, I'd roll a D100 again to determine how many arrows come at them with each arrow randomly targeting one of the party members. The attack and damage from each arrow would be the same as the archer warrior stat block, but -8 for the attack.
5.) Determine how many of the 350 ally fighters die from the arrows and boulders by simply rolling a D100. That's how many die. Do the exact same for the incoming army to see how many of them die from your own allies' archers. Not perfect but it's simple.
6.) Now for the invasion. Let the PCs be creative if they want, but regardless the army attacks in waves. I'd have the PCs start by dealing with a total of 10 melee soldiers, 3 adepts, and 3 archers. After every round, an additional 3 melee soldiers, 1 adept, and 1 archer all join the fight. This represents the invasion coming in waves, and a portion of the army targeting the party.
7.) Also at the start of each round, roll 2 D10s. The first d10 represents the number of enemy soldiers who die at the hands of the allies' army. The other d10 represents the number of ally soldiers who die at the hands of the enemy soldiers. Make sure to paint the scene of the battles and screams and destruction going on around you during each round. :)
Just do this until either the party dies or all of the enemy soldiers have died (be sure to keep a running tally!!!). Yeah... the party will probably die. :p But since they weren't MEANT to participate in this battle and they had ample time/opportunity to get out, they brought it on themselves.