The game is fun for a few moments. It sure looks nice, I especially like the screen transitions. The premise is simple, but good enough. I don't want to criticize your game (at least in a mean way) but it did get boring after a while. I wonder if the game speeds up as time progresses. Because it surely doesn't feel that way. This (pretty small) change would have made the experience so much better. That is the main problem for me. And it also would be probably better if the player couldn't shoot all the time. You just end up holding the trigger all the time and showering every enemy with bullets. Because sadly, players take the easy way, not the fun way.
I hope that I didn't offend you in any way. I'm not good at writing critiques.