This is a good game
I find this is more of a mini-game than an actual game. I'm sure you've gotten tired of hearing this, but the game is a bit too short. Just as you're getting into it, it ends.
If you make a sequel, I have a few ideas:
1. Add some different backgrounds (i.e. lava, underwater etc.)
2. Make it more challenge-based. For example, you could have levels where you have to blow up as few robots as possible, instead, or have levels with enemies that you're not meant to destroy...if you blow them up, it's game over. You know...stuff like that.
3. Change the music or add a mute button (if there was one I didn't see it)
Good time-waster though. 4/5 8/10.