If you take away all the walls and siege equipment, all you have left is a field battle, in terms of mechanics.
Enemy reinforcements spawn just behind the gatehouse, just like in warband.
Once you get over the walls and all your soldiers are inside, basically all you do is sit on top of this spawn point and slash until there are no enemies left to spawn.
My Suggestions:
Siege should be about capturing battlements, rather than killing everything that spawns.
Put flags on battlements and on towers that can be captured, which triggers enemies to organize a counter-attack to recapture lost battlement.
Spawn enemy reinforcements inside towers and/or deep in town.
Stop spawning in enemy reinforcements if only 1 or 2 capture points left. Rout defenders, or have them retreat to barricaded streets.
Don't try to spawn all the defenders during the battle, it's unnecessary. When attackers capture everything, all the remaining defenders can become prisoner.
Can you add a command to the command option to raise or lower the shield? Like the archery command of the bowman. Now the bowmen are very powerful, and the infantry will often not lift their own shields except in shield wall formations, especially when using a ring formation against the Riding...