Really nice game! The music is beautiful and atmosphere, never got boring, and the levels are fun and extremely challenging after a while. Level 7 was a big difficulty spike for me, and level 18 was hell; I kept thinking to myself, "How horrible is the next level going to be?" Thankfully, that was the last level.
The difficulty was pretty insane and unforgiving, but that is to its benefit. It works because the restart button is so quick. (I praise this very highly, as there are quite a few die-a-lot rage games that DO NOT implement an effective and fast restart button, which just makes the game tedious and annoying.)
Great game, and a lot of kudos to you. Keep up the great work.