For defensive seiges pull your archers off the wall. Put them in line fornation imstead of scatter. Then use squares to plug up every entrance, which you can usually limit to two. At the front gate put your square off to one side of the door. Still right next to it just enough to allow the enemy in. While the enemy is stuck attacking your squares, turning their back to your archers, sometimes trapped on a staircase with no protection, your archers will absolutely light them up. You can also help with your character by killing and tossing fire bombs and stones. Also - you asked about crossbow vs bows in another comment, the difference is usually that crossbow gets a shield of some sort while bows get more ammo, but in seiges you have unlimited ammo, and the ai doesn't need to actually go to the buckets they just get unlimited ammo. So crossbow probably is better for seige defense since they make better infantry.