When I played it for a bit I was a bout to close the game because I felt I had seen everything, but then suddenly those slug-luke enemies spawned. The same happened a bit later with the squid enemies that shoot those projectiles. Unfortunately I died there, and I did not feel like going through the whole game again to see if there is even more content. Did you have any more enemy types?
I thought the game was a lot of fun and very chaotic near the end of my playthrough, with the squid enemies, the slug enemies and the normal enemies all together. I agree that you should not spawn all of them straight from the beginning, but I believe the game would be more enjoyable if you get to that point a better more quickly. Now the beginning is (at least for me) pretty easy and boring, but you have to get through it to get to the good stuff. :/
The art was simple, but looked really good! Maybe some explosions when you killed enemies would be a nice touch, but that is nothing serieous. The music also was great! Did you make it yourself? If so, I am impressed! :O I always wanted to make my own music for my games, but I can never make a tune I am happy with.
Sorry for the long comment. I really enjoyed playing! Great job :D