terrain, with enough dips and rises in the ground to provide some cover. Hills are difficult for moth troops to navigate. Dwarves, trolls, and orcs have enough familiarity with the terrain that they can pass through it without being slowed down. Cavalry have enough trouble navigating the terrain that any defensive aid lent by cover is negated.
Most units have about 50% defense in hills, whereas cavalry are limited to 40%. Dwarves enjoy 60% defense in hills.