[Agema]

Survivors from the losing side of a siege isn't that improbable. During the rout some could escape the fortress outright through tunnels, sewers, maybe a river or the sea if there's one available. Some might play dead, or hide, remove their uniforms and pretend to be non-combatants, then rejoin the cause when it's safe.

In terms of famous survivors from a siege in myth (and Dom3 is surely based on myths!), Aeneas and a load of Trojans escaped Troy to later found the progenitor of Rome.

Edit:
Actually, I should add: if you want additional casualties after a siege, you should give an additional morale bonus to the defenders. If their chance of dying after routing is increased, you can bet they'd fight a lot harder too.