Siege defence is totally broken. Units are not taking positions on the walls and even if i put my own army on the walls they are not attacking (ranged units not shooting). No one is using siege equipment or siege defence equipment. I can send You a save file just before a siege that is always like this (tried reloading 10 times). On the other hand i had no problems when attacking (just one enemy archer stuck in gate).
Got same problem, and want to ask, how u started fight.
Cause when town was under siege I attacked on camp(on map) and choose "Help *TOWN_NAME*" and got same problem. I noticed that troops spawned like standart fight, but in town, and they don't use siege euip cause they use "default" war script. If u look, all of them try to get through gates, and no siege machines.
And I saw a stream, when guy was stucked in a city with troops and defence siege worked fine.
So, other words, when u attack siege camp, u should attack not in town, but in forest or smth, but u spawned in town, cause you choose to help town. And script works like default fight, not siege.