Mountains from AW1 and AW2
Land Vehicles may not traverse them at all, and must find a way around any mountains in their way. Indirect Combat units may fire over them without issue, however, so they provide excellent defensive terrain for Indirect units.
Another benefit of Mountains is in Fog of War games. Any foot soldiers on mountains will have their vision increased from 2 to 5 so long as they are on the mountain, providing high vision on the spot should the need arise.