Really fun and addictive game. Loved its mechanics and graphics.
I agree with Mayo and DeepBlueMax: Having a short and skippable cutscene or a visual cue, like a small crosshair, would be really helpful (at least at the start of the game, you could get rid of it after the player reaches certain score) since it took me a while to to figure out what I was exactly doing to aim at the enemies correctly. Hope you keep working on this! It has lots of potential!