I think a big part of the problem lies with the AI more than the castles themselves. The A.I. do not use these castles in a strategic way. Part of the problem is actually the army system that has become so integral to the game. These armies can suck up almost all the lords to march to some far away fief, leaving the borders utterly defenseless. Lords do not defend fiefs, a tie in with the issue of not being able to enter beseiged fiefs. Instead the A.I. will just waste their time standing outside the castle looking on and not being able to do anything. And when the battle starts, they just wander back off into the wilderness, their effectiveness during the time of the siege being nill. Nothing defended, no siege hindered, literally nothing accomplished. It's not efficient in any way. Lord's should hang out around border castles and instead of ridiculously tailing armies they are never going to fight, they should enter the border fiefs to try and deter the invading army or provide additional defense, in the least.