Great game! I really like the gameplay, art, and music
A few suggestions I have from playing it:
The shooting itself felt nice, although it might be better if the direction the player is facing could be influenced by the mouse instead of the keyboard since shooting a bullet behind you looks a little unnatural.
You might want to consider increasing the screen size a little, because by the time you see an enemy it has already shot at you, giving you very little time to react.
The hitboxes on the main enemy type also are a little small, I've had a few bullets fly through the top of their heads.
This is probably already part of your plan, but it would add a bit of polish if the level end square had a texture, even if it were just a ball or something.
Overall the game works really well and was fun to play, good luck with updating it!