Forts work similar as in RTW. Blocking every access road to your provinces can prolongue peace with a faction (maybe it changes the pathfinding of the AI, whatever it works). As garrison 1 cheap unit will be enough. A peasent unit that is already damaged in half or any other badly damaged unit will work best.
If an other faction finally decides to wage war against you and besieges the fort attack with the besieged unit (might aswell autobattle there). You loose the battle and the enemy sits within your fort. Now all you need to do is sally forth and besiege the fort in return. It does not matter if the enemy gives up after 3 turns or if you exterminate all of them in a glorious fort battle, noone will escape (make no prisoners )