Good idea! I feel like it needs a bit of polish though:
- The bullets are not appearing.
- You can run into walls (make sure to apply Physics to your sprites, I ran into the same issue).
- In some places enemies are appearing right next to the player, very rage inducing mechanic :D.
- When you restart, there are no enemies and the camera is not attached to the player anymore. You have to entirely restart the game for it to work correctly.
- Heart unlock did not work correctly for me: it took a heart out instead of giving one
- The controls are not clear, maybe you can add tooltips into the game.
With more levels, a few more enemies and different types of weapons, you'd have a very nice game! The art and in-game music are solid!
Keep up the good work!