Just another note on stone wall sieges, keep your infantry away from the seige towers until they are at the walls, or you will get hammered by the ballistas they carry. I find keeping the infantry on the next section of wall ,just the other side of a stone tower keeps them close enough to rush over when it gets there. You might as well not bother with attacking seige towers with archers. keep them around the gate(s) to attack rams then once they are all out of action just pick off the enemy as they run around the walls to get to the seige tower. once the reinforcements are being sent to the tower you can usually rout the unit before it even gets close to your infantry!