Nice retro shooter. As you must know, some sound/music will add a lot.
The game feel good, but the relation between your strength and the number of ennemy is not trivial (at first I was thinking it was related to your life). It perhaps need some gauge that fill when ennemy arrive, and empty when ennemy die. And when it's completly fill, let you choose an upgrade (fire rate/speed/armor, etc...)
But I know 48H it's really short.
Well done for this short amount of time.