Very well-executed concept, and very good use of your assets. Game was tough, but managed to get to 700 after a few tries. Very high marks for this entry!
This didn't effect my enjoyment, but just so you know you've got a minor glitch: your and the enemy's bullets behave differently at different framerates (play the game with your laptop unplugged and you'll see what I mean). You might be aware of this, but all you have to do to patch this out is make sure you multiply your bullet's speed by delta.time.