Rivers and bridges
Big rivers slow armies down. Bridges are the quick way across.
Major rivers are drawn on the map. Armies cross quickly at bridges, or wade across anywhere else, which is slow and leaves them weaker in a fight while they cross.
Enemy forces near a bridge block it for you, so your armies must go around, ford the river, or fight for it.
Bridges are destroyed by missiles, by engineers, or when the side holding one loses the battle to defend it. A broken bridge stays broken until engineers on the spot repair it, and someone pays for the work.