A cute little game. It isn't a very complicated game but does what it need to do.
possible additions / tips:
- add a little bit of screen shake.
- I couldn't find the 4th key. it was a little bit confusing.
- you could add an extra level maybe even an boss battle. (the game is kind of small)
- make your player shoot faster. you might have to re balance the game but fast game-play wouldn't be a bad idea.
I hope my advice is useful. Keep up the good work!