[Digress]

I think retreat from a besieged fort should either be impossible or at the very least fraught with danger.

Some percentage chance of escape after the castle has been stormed based on the patrol strength of the besieging army perhaps.

You could also argue that non-stealth troops/leaders should be able to withdraw before the castle is stormed, taking the risk that they will be discovered be the besieging army whilst making their escap. This would again be based on the patrol strength of the besieging army - perhaps with bonuses accrueing with the length of the siege.