Siege towers definitely need work. Right now, ladders seem more effective. They should either be ramps instead of ladders or the ai should have enough space and programming to load up on top of the tower before the tower gate opens. When 3 ladders are that close together, the defending ai dogpiles on top of it and will massacre anyone using it. The default ladders are much safer since the ai has to spread between the 3 different points.
That said, so far the best way to break through a siege that I have found is to have everyone follow me, break down the front gate with a ram (the ai very rarely seems able to break these even with full siege weapon set up) and then just run past the guards behind the second gate with all the troops following me. This works because there seems to be two additional problems with siege defender ai:
1) the ai defenders right behind the main gate will just stand there. I don't know if they just don't have enough room to attack or what, but I can regularly run right by/through them and not get hit at all. The ones on the edges will slowly fight back but the ones in the center usually stand there waiting to die.
2) The ai never seems to utilize the kill box between the main gate and the second gate. I don't think I have been killed in there once. It is essentially a safe place for attackers when it should be one of the most dangerous places to be. To be fair, defender ai in general is a problem and one of the hardest parts of being on the defensive side of a siege (half the time my troops won't even get on the catapults or ballista no matter what I click).
The sieges look great and the foundation is there for epic gameplay, but there definitely needs to be some work done here. Maybe custom orders or some different plans that can be given pre-siege to alter behavior away from the field behavior.