Hi Raijer,
I understand your feelings, sieges are heavily biased in favour of the defender in all TW games. However, using loose formation for troops, or testudo for the Romans in the later game should you be discussing R:TW, really does help restrict the number of own casualties to a minimum.

Another tactic you could use involves the use of peasants as a suicide force. Simply line them up infront of the walls, in loose formation, and let the towers batter them! While the arrows are focusing on them move in your real force to do the proper work! It's also often wise to use peasants for ramming and brining the siege weapons to the wall. Afterwards you can order their withdrawal and again send in the real men to do the full fighting.

Hope this helps you, cheers!