I really like playing Civ5, when a city is behind a mountain, if properly guarded, it's nearly impenetrable.
A strategically placed city on choke points can be a shield of your whole empire. Enemy settlers or scouts can't access a vast majority of land. Also people have fun with canal cities with trading routes shortcuts, etc.
However games like civ5 only have mountains, the river tactics is weak, historically many battles were carried over river and controlling a bridge is vital.
Are there any games that focus on terrain tactics? I think combined with some aspects from Tower of Defense, the game can be quite fun.
I know games like Total War where terrain plays an important role, but it's more campaign focused. RTS is too short-lived.