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Developed by the Discovery Channel, Cannon Challenge places you in the control of a Non-Line-Of-Sight Cannon. What you do in this game is control the elevation and velocity of your gun to precisely hit the targets placed all over the screen. The game is pretty easy to get into and can be addicting at times as well.
There isn't really a lot of movement or iPhone specific enhancement used in this game. You don't do anything besides move the two dials to the specific calibration that you wish to shoot your gun at. There isn't any use of the accelerometer or other aspects of the touch screen either. So if you're looking for an iPhone-centric game, this isn't going to be the game for you.
The challenge in this game comes from the learning curve that you need to overcome initially. The game itself just requires you to hit an ever increasing number of targets, some of which will actually be moving. But these targets become easier to hit once you learn exactly how much you need to turn each dial. When you've gotten enough of a grasp on the dial to hit a target on your first shot, you've pretty much exceeded the fun capacity for this game.
Considering it's free, you should definitely check out Cannon Challenge. It will keep you going for awhile without getting bored. Plus you can exit the game at just about any time you like, making it very easy for you to pick it up and put it down as time permits.