In the Layer menu, select Terrain to make sure you are in the Terrain layer.
Note: Setting your brush to a terrain type using the Tool Tree or Brush Palette automatically puts you in the Terrain layer.
In the main window, you can click to place individual terrain tiles on the map or click and drag to fill in large areas with the currently selected terrain type. If you hold the cursor over a particular location, you can click multiple times to cycle through any artwork variations for that terrain type.
Terrain is either passable or impassable:
Passable: You can place units on this terrain, and units can cross this terrain in the game. This includes terrain such as dirt, grass and concrete.
Impassable: You cannot place units on this terrain, and units cannot cross this terrain in the game. This includes terrain such as cliffs, walls and water.
Terrain is also buildable or unbuildable:
Buildable: You can place buildings on this terrain, and players can build on this terrain in the game.
Unbuildable: You cannot place buildings on this terrain, and players cannot build on this terrain in the game.
Note: Doodads and Units can be affected by changes to the terran underneath them. For example, if you change the terrain beneath a building to an unbuildable terrain type, the building will be removed.