I really like the idea of making a tower defence game without a tower, but it's really hard to feel into the game as some important assets are missing (enemies, own character) and sound could also contribute. The existing assets look great, although I'm not sure if the background tiles have a different resolution as they look a bit blurred. However, I think it's very good that you show the controls before starting the game, as it helps the player a lot. Unfortunately, I had a bug where there were always enemies in the centre for a very short time and the waves took a very long time to trigger, even though all the enemies were defeated.
But all in all a cool concept!