AJ11
Emperor
Otherwise, forts can force invading forces onto flatlands, or to expend tremendous resources attacking the fort that has a handful of units inside (IE losing 2:1 or much worse generally, just don't put too much in the fort to be vulnerable to collateral and they'll have to throw away a ton)
Which is realistic. The Alamo springs to mind. As does Guam, Saipan and Guadacanal.