There are numerous terrain types that also have a huge influence on the battle. Some battlegrounds are plain fields, good for Cavalry and Archers. Units can hide in the trees, move faster on roads, while the open ground suits units fighting in special formations.
Forests are areas where units cannot keep tight formations, and so they will automatically change into Horde formation. Forests will also impede both the movement speed of units, and their combat effectiveness. Light Infantry suffers smaller penalties than Heavy Infantry or Cavalry. You can choose to highlight all forested areas in the Strategy Options.