1. change fortification so it has health, and the less health it has the more plunder you get(see darkthrone), and
2.Make it so when your fortress health is at 0 you can assasinate their king, when you assasinate their king you get their empire and they restart(getting their empire means you get all their weapons and all their soldiers, sentries and spies, you also get their siege tech and their fortress if it's better then yours.