Siege towers need a platform over the drawbridge level that can be manned with archers shooting at the defenders.
Yes this would be a cool idea. One option would be to have levels of siege towers, with a more advanced one manned by archers and that is taller than the enemy walls (yes siege towers were at times built taller than the enemy walls).
I still think they should change back siege towers to use ramps like in Warband, sure it would mean infantry can rush up on the wall but I feel like that's the way its supposed to, now the only benefit the towers really gave was that it provides slightly more protection for troops and some more dumbness from the enemy climbing down them.
In theory, if the AI used all 3 ladders of a siege tower, using the siege tower would be an better than ladders, but that isn't happening right now.
I do agree the siege AI needs work.