Nice, although I'm not sure why it's free only for a limited time. (But thanks.) I saw a request on App Store to make this full screen. If you opt to go in that direction, please offer a choice between full screen and window-only mode. I do not like full screen.
Any chance you plan to do Space Invaders? I've seen variations on that, but none that are actually close to the real deal. I would definitely pay for that.