Warcraft III takes place on a map of varying size, such as large plains and fields, with terrain features like rivers, mountains, seas, or cliffs. In Campaign mode, the map is initially covered with the Black Mask, an impenetrable covering. The Mask is removed from areas that have been explored, but those that are no longer within sight range of an allied unit or building are instead covered with the fog of war; though terrain remains visible, changes such as enemy troop movements and building construction are unseen. During a game, players must establish settlements to gain resources, defend against other players, and train units to explore the map and attack computer controlled foes. There are three main resources that are managed in Warcraft III: gold, lumber, and food. The first two are required to construct units and buildings, while food restricts the maximum number of units the player may control at one time.