This game is absolutely beautiful!
I agree with the other commenter that a more defined grid would help line up shots.
I also think it would've helped if you clarified that the side facing the enemies had to line up numerically to the enemies, not the side facing upwards. I didn't get that till my third run.
I think you could do cool things with this! Add power-ups and ammo caches to keep the game going!
Overall great job :)