Capturable Units

My suggestion is simple: Make certain units that, when they die in battle, reappear in the possession of the side that won.

For example: Country A has a fort in City A. Country B attacks City A and overwhelms the defenses. Rather than the fort being killed, it instead reappears in the possesion of Country B. Likewise, if Country A won, the fort would return to their possesion (maybe youcould make capturable units fight last to prevent bloodless victories for the defenders).

Maybe only if the city had no other units in it when captured.

If there are other defending units in the city then capturable unit would get destroyed as just capturing it seems OP