Very nice little addictive game :)
A few thoughts:
-The instruction screen is complete but a bit too dense of text, it is a bit heavy to read. I think it is manageable to convey the same amount of information with less text and images
-I often die because I don't see the gun projectiles, they could be a bit bigger.
I really dig the graphics by the way, it is inspiring to see such a nice game with this minimal graphic style.