Your game puts a cool twist on a familiar classic concept, and I love that. In its simplicity, you have even achieved a sense of exploration and discovery.
To critique, I will say that your game could use a touch more affordance. I discovered the power-up screen by accident, and was 2/3 through my timer before I realized that shooting things got me points.
Other than that, your game had tight controls, an intuitive navigation system, and a very well-unified aesthetic.