Lovely game, but I did have a few points of critique I feel would help you develop the game further:
1. If you shoot at a fixed rate, and always want to be shooting, then having to hold down the mouse button is pointless. Just autofire, or change it so that there is a reason for the player to want to stop shooting
2. More sounds would have been a great addition to the atmosphere of the game
3. I liked the art, but I feel like a few more sprite for more animations would have definitely helped.
I think that with more time, this would have been an absolute gem.