Enjoyed it, but have a little constructive criticism.
First, I'd recommend watching this amazing talk by Vlambeer. They have some incredible tips on polish that you might find helpful.
I would do something to differentiate enemies and the environment; as they're all neon cubes, you can't really tell them apart. Personally, I'd either fill in the environment, or make it monochrome.
Sound would be an improvement: not necessarily music, but you'd be surprised how much of a difference a simple beep playing when you shoot makes.
Finally, because enemies don't collide, they tend to form an overlapping bunch. To fix this, I'd either make them collide with each other, or (and this is more of an advanced topic) read up on flocking behaviour.