Sally fight: Instead of waiting for the force of 200ish guys to build ladders/ siege the town, they rush out of the gates to swamp you with the 600 they have. Your not fighting in the town, but outside right near the gate(sadly closed), and for whatever reason they don't man archers on the walls. Because of their number advantage they would swamp you, but if the battle counter is 100 then it turns into a fair brawl. The archers I mentioned before are at the base of the wall, where all of your units can attack.
So in order on what to do:
Take out all of the lords you can, use whatever army you want.
Gather a composition of 60% archers, 40% heavy infantry/knights for the actual siege
Before you go in to build ladders/ towers, set the battle counter to 100 (Or lowest)
When the ladders are done, they will sally out to attack you(Ground battle, no walls, no horses, hence why you want heavy infantry at the start)
Once the sally battle has been won, you can attack the walls (Such as normally)
Use the archers you brought to rain arrows on the walls. DON'T send in infantry,use them as a shield for the archers.
Retreat/repeat until their forces are at around 200, might want to refresh the siege for the final push
Use infantry/knights to rush, once they get on the walls rush your archers. If the wall has steps leading up to the wall, thats where the brawl will lead. Plant the archers to fire in that area.
Should be able to take towns that way, that's what I did for 5-6 of them. If you still have questions ask em