Nice retro top-down shooter with a short self-contained narrative!
Music would've been a nice addition to the game as others have pointed out already.
I would've liked some more feedback regarding the boss and mid-boss enemies. An HP bar for the final boss would've been nice.
It was pretty easy to clear the game with barely one or two heals, so you should probably spend some time fine tuning and balancing the difficulty later.
Overall a nice entry!