Also if your playing native,which im guessing you are by the sounds of the convo. I would use all archers or all crossbowman that are maxed. I would use a crossbow or bow my self. When the battle begins make your guys hold postion. pick off as many as you can by your self. Let all the guys run out of ammo before allowing them to go in. I know on some of the seiges the ramps are not placed correctly and your men will bunch up at the top and get picked off by archers on the sides. If this happens I call my men back down and go up my self and pick off a few to unblock them so more men will move to the front for your guys to fight.
Also even on the ones with a good ramp I noticed that sometimes they still get stuck at the top and I have to actually go into the castle and make them follow me to get them in then I give them the orders to charge.
But I almost always have to retreat let my men refresh let my own ammo restock to pull off a siege. Heck I have 180 men and im still having to do that.
They also pointed out on the surgery skill is most useful in this sittuation. when you retreat you come back in fullly restocked and so do your men,even the ones that were knock unconsious.
Also when you retreat you can then leave rearm your self with a sword or shield or weapon of your choice. when I go in I have my crossbow and 3 large bags of +2 bolts.
Hope that helps.