In a recent siege, routed defenders tried to get up to higher level.
If storming a castle you'd have to do 2-3 (depending on castle level) succesful storms, it'd make for great opportunity to inflict heavy casualties, as well as have some sort of plan to enact other than headshotting top tier enemies while your lemming-like troops keep jumping/being pushed off the walls.
I've managed to offload my treasure and prisoners to different castle, and with select 120 troops inflicted 300+ dead on enemy army, which was quickly defeated. My castle fell, but I'd say that was still very effective defence. And if I could retreat my troops in orderly fashion to upper castle for stage 2, I'm pretty sure I'd hold if AI wasn't acting like Wallstreet gamblers after a market dip.