a couple of points - why dod you attack the rebels? they are muich less agressive than faction armies and i have only seen a rebel armyt lay seige once in over a month of solid playng and that was in a modded game.

secondly - are you actually fighting the seiges or autoresolving them? it is possible successfully defend a seige against massive odds. do thew obvious things -

line the walls facing the attacker with archers.

if they bring ladders or seige towers place some infantry units on the wall in the section the are going to hit. cheapest ind will be fine - you can nearlly always successfully repel a wall attack.

place ~ 4 units of spearmen by the gate. if and when they breach the gate with a ram rush forward and plug the gap with your spears. create a bottlekneck here and you will achieve mass slaughter - and often kill the enemy general as he charges into your spears.

if they smash holes in the wall with seige equipment plug them with spears. they rarely seem to use the ram and seige equipment at the same time. so some of the spearmen guarding the gate can be used for this.