It's possible, but I don't think it likely. Trying to program that into the game would add another layer of complexity,and I doubt CA has neither the time nor inclination to code such a feature.

I think it's more likely that the citadel will be the defenders' final refuge in a battle, where they will make their last stand. I imagine that as an attacker, you will have to take a city's citadel, else you will not conquer the settlement.