The most important impact terrain has is how it affects movement of armies. Even the most powerful army can be undone if it's outmaneuvered by a more powerful adversary. Additionally, some kinds of terrain cause attrition.
Hilly light forest
Terrain features on the battlefield can play a decisive role in any battle. Hills, mountains, forests and rivers can be used to conceal troops or funnel an enemy’s advance.
Most troop-types can be hidden in forests, as long as they walk or remain stationary.
Units with the high ground enjoy a significant advantage when fighting on sloped terrain.
Steep slopes are impassable, and may be used to cover the flank of an army.
Hills and other terrain features can obscure visibility, allowing units to be hidden from the enemy’s line of sight.