Glad you enjoyed the art! But in fact all the mountains are made using a 4x4 tile set, the trees in the background are a layered 4x4 tile set, and the castle walls are also a tile set . The only assets in the game that do not fit on a 4x4 and are not a tile set are the player/enemy animations, the blacksmith/blacksmith's house, and the graveyards.
Here's an example of how the mountains are made: 