Impassable terrain is terrain so difficult to move through that it cannot be crossed during the course of a battle. It includes terrain features such as rivers, lakes, impenetrable swamps and sheer cliffs. Troops must go round impassable terrain.

Players may decide before battle is joined whether certain terrain features will be difficult, very difficult or impassable during the course of the battle. The method of generating scenery already described can be used to determine whether scenery is difficult or very difficult, but players can always agree to make terrain feature more or less passable if both sides prefer to.