Very nice execution of the "one bullet" idea, i really enjoyed the large variety of enemy design. The large fire ball really made for some challenging levels but sometimes it would get stuck bouncing from one side of the screen to another so my suggestion would be some way to change the bullet directory after you've cast it.
I think there's something funky going on with the movement, it looks to be grid based but it does not seem to line up with the floor tiles, making it kind of counter intuitive.