I liked the physics and the way you implemented the small planet. The game looked visually pretty good too. I got to wave 15 before I ran out of ammo and had to resort to punching the enemies with the gun.
Reload animations felt crispy, the movement was fun and it had a great menu screen. It was hard to see enemy due to the low resolution. I had a lot more fun playing it than I thought I would.
I really liked the atmosphere, the effects, art style and audio worked well together. Though there was not as much to do in terms of gameplay. Also, I got the good ending first try :)