Fun to play, and the difficulty build-up in the levels is nice, but the levels start very slow.
I would consider speeding up the initial speed of the game.
The bats were not very easy to spot with the mountains background.
I agree with the comment of someone else that the background scrolling is a bit strange, I even felt a bit dizzy after playing, but that might be on me :).
I really liked that you created the audio by yourself.