This was a nice and compact little game, very clean and simple. As others have noted, some sound effects and music would go a long way towards making it feel more complete, more player feedback on bullet hits would be very helpful, and some indications on what increases your level and health would be great. But overall, great work on making it so small!

Also congratulations on optimising the game so much. It runs pretty smooth.

Its a very interesting game and I like the rectangle look of the game. Perhaps if you could add some sfx it would become better.

I wanted to ask about engine but I see comment bellow has already done ! Nice little game ! I think it would be more impacting if you add feedback when you hit ennemies or when you lvl up. Reduce rate fire of red ennemies could be good too. I want to kill them ! :')