If you quickly attack them you can wipe them in 100 days
i started over a new game with an optimized charisma & agility (loot) based character.
after 20 days my army was 105 strong and ready to defeat anything with 50 master bowmen some rhodok sharpshooters etc.
and without joining a lord just by hunting down bandits of all kind. i purchased a factory in each town.
and hopefully because the dark knights invaded my rhodok day 60!
They have captured 3 towns and a castle you have to almost follow them and recapture the town again and again, in this game i have captured each towns 20 times the first battle was a litle rough as i was not dealing a lot of damage to them but the master bowmen and sharpshooters will clean the walls in no time, if the DK remain entranched you have to climb the ladder to bring them so your ranged weapons can kill them, watchout at time they may use the ladder to charge your troops (if they charge jump off the ladder, you will take some fall damage but its better than a dk blade in your skull)so keep the melee soldiers right in front of the ladders and give them the order to charge so they can keep the DK on the ladder, your ranged weapons will turkey shot them on their ladder.
loot, sell (if the vendor only have 800gold left and you sword can be solded 1500, sell it as you will have too much stuff to sell anyway)and store the best stuff in the town chest so you can use it later, don't worry they don't steal the stored stuff after recapturing the town. keep 0 to 1 man in garison because you will loose the town and go capture another when they will besiege the town you have just captured, over time you will weaken them and their lords will need to rest in a town, besiege the town, capture the lord (i let him go)
today we are day 170 they have no town or castle left so their lords can't respawn but i have still Lady larking and her 600 strong army roaming the land, she don't capture anything but she is still looting Swabian's villages, if she ever attack my main town i will be ready to defeat her with my 230 army and a 400 strong garrison. i have a bunch of rhodoks arsmen and spearmen to tank and an army of sharpshooters.
If you have to defend a town, find a safe place so you can't be shot by their bowmen and use a bow, don't aim just shoot like a "machine gun" where they heads firt appear when they reach the wall, don't forget to build a blacksmith as you will need infinite ammo. ^^
if they are using a siege tower, you are doomed so don't try to defend that town, those should be the last one to captured when they are already weakened.
equipe your heroes with DK stuff as soon as their strengh reach 10/12/15/20 for your heroes raise their strengh strengh and only their strengh up to 20 all your heroses must use a ranged weapon, bow or xbow.
I have almost all heroses but allysa your herose don't like her much obviously.