A few quick notes for 1.4.1.
If you go into a town with an army before it is sieged and the enemy army sieges it, your allied lords that are in your army do not join the defense.
Defenders do not use all defensive siege.
A lot of catapult placements cannot hit anything as a defender due to overshooting the entire field and there doesn't seem to be a way to adjust the power of the shot.
A bounced catapult shot does not do damage, although it still has a lot of velocity.
Defending units constantly fall to their death when defending against siege towers, defending units should be prohibited from getting on the siege tower from the castle wall. Attacking units falling off the siege equipment is totally fine.
Castle garrison/militia for castles that you own do not operate under your command and are uncontrollable.
Now for unimplemented things:
Attacking AI does not react to any sally force in the siege map, you can sally your gate defenders in the path of the ram and stall the ram forever (attacking AI sends units 6 at a time to move the ram and they can get mowed down before they reach the ram and the AI wont clear the blocking troops. If the attacking army also does not have siege towers, an attack can be forestalled for a long time with this tactic as the ladders will not be used unless the ram reaches the gate, or if the ram is destroyed.
Commands for sieges seems entirely unimplemented, you cannot order units to use siege, defend the walls etc.