I personally dont mind the castle-looking tileset. However, from the way the rest of the screens look, you're gonna need some decorations.
Notice that all of the tilesets in the editor have scenery in them.
Use it.Also, make sure your levels aren't flat. Outdoor scenes should have hills, cliffs, and all sorts of things that aren't flat.
Just don't make them flat for god's sake.Levels need more than a floor, walls, and objects. Add some background and foreground foliage, decorations, mountains, waterfalls, lakes, etc., and much more non-flatness, and this level will be a
lot more interesting.
Good luck.
If you need more tilesets, use
these