Fun but a bit hard to see
The game is good, and there's plenty variaton in enemy types and behaviours, but some of them are a bit hard to see. The 'whirlwind' enemy is almost insvisible, and the large M-shaped that shoots should have bullets that are easier to see (took me a while to figure out it was shooting. Checkered backgrounds are difficult, they almost require that all sprites have both a dark and a bright outline to retain visibility as they move over them. Keep it up, though, a few changes to the visuals and this game could rock.