From my experience, you need at least two catapults to sufficiently destroy a small castle with stone walls. Though it is possible to just blast a hole in a wall and go from there, expect to take many casualties. Ideally, you should destroy as many towers as you can, and blast several holes in the walls before sending your cheap units in.

Sieges have so much potential to be very strategic battles. I would have appreciated the ability to capture parts of a castle, and fight on the walls. But that was done in RTW, so I guess I shouldn't be complaining.

Does anyone know what effect destroying a keep has? I used to always go for the keep, thinking it might weaken the defences somehow...