Very cool and unique entry, I love it! The music is wonderful and helps you get "in the zone" while playing. It would be even better with sound effects, but even without it the music compensate for them!
I love the "triangle" art style and the overall polish. Gameplay wise, it's easy at start but become quite challenging once you reach level 3!
One suggestion would be to reduce the number of hit points of the enemy on the lower level, especially on level 1 - as the game is meant to be played over and over, after a while it get tedious to restart knowing that you'll have to kill them all again has they have a quite large life bar!
Anyway, great work on this game, and I hope you'll keep working on it!