It's a solid mindless shooter, something that could work on mobile very well. I liked the art, while simple, it accomplishes the end goal of establishing the environment. You could include some variation in terms of power ups, or differing guns like a shotgun or plasma rifle.
Enemy variation would be a nice to have, if the not in terms of mechanics, then in design variation to spice up visuals and keep the player interested.
All in all a great game for a lunch break playthrough.