Terrain

A tile of terrain has a height and a angle of steepness based on it. Different units can cross different steepness levels.

You can set the property of a terrain as not being ground. Open terrains/tile_sets/Haven.til.xml. Look for the line that says: <TileProperties ground="false" open="false"/> If ground="false", then ground units can't move on the tile. If open="false", then nothing can go on that tile (they have to go over).