The Mountain Tile is a type of tile that contains an immovable mountain. Nothing can be built on it, and Tents can not generate on it. It's only use is as a barricade as a Tower can not pass it with their attack, and it takes much longer than a Normal Tile or Forest to claim. It is technically possible for your Capital to be completely surround by Mountain tiles, in which case it would be impossible for other Empires to claim it using a Tower. Also mountain tiles take the same amount of time to capture as towers.