If you have a lot of influence this is something you can do:

Raise an army of about 1200 - 2000 men

Begin a siege and start to build trebuchets, once a trebuchet is built pause the game click on the trebuchet and reserve it. You do this with all four trebuchets. If you have a lot of time you can reserve 4 of each siege weapon. After you have all of the trebuchets built place them all at once and watch the enemy siege weapons break apart like a house of cards. Once there are no enemy siege weapons left you can send troops/attack whichever you prefer.

This method is basically a guaranteed way to take any town or castle.

You will have to watch your influence though and make sure to add cohesion once it gets low. One more thing is that you should probably be on the higher end of 1200 - 2000 men if your enemy is a large nation.