Another way to force a retreat onto the heroes (other than being overpowered by the Shadows); two or three important people to the heroes could get wounded + kidnapped (always two delightful words to combo together) by the Shadows, and the Shadows agree to do them no harm and heal them too if the heroes forfeit control of the city. Also offers up the opps for extensive ransom speeches and confusing shennanigary.
I really have no idea how to transport that many characters out of such a situation without killing/separating large groups of them. *shrug*
@Giga: I have no idea if I'll make another character, it entirely depends on how my boat is floating and if I too desire to kill one of my children. Which I might. I very well might.