One possibility would be a scene, where a small strike force from your party tries to clear a way to get in, and has to fight some of the besiegers to do so. The idea being, that it's a surprise attack, before the besiegers can muster a larger force. It would have to be balanced along the relative party sizes and also depend on tactics skill. Potential losses would then be taken directly.
This option might become too powerful though, as players could perhaps make disproportionatee use of it, comparedto AI.
I'd also say that castle defenders should join the fight when the besiegers are attacked, maybe arriving with a little delay and starting at another point of the field.