Guard towers take up one tile, but will overlap about half a tile in a 3x3 tile area around the central tile.
When attaching the last piece, try standing a bit diagonallymake sure you are facing the tile you wish the tower to be created in. Guard towers can be rotated now, to place so the tower more aligned initial rotation doesn't matter. But they are locked to the tilethey are created in.
Do not build fences or a structure on the tile borders of the center tile of a guard tower. Although the fence may build, it will eventually be destroyed.