To use the game, print out a special PDF template on any printer, trim to size, and place it on any flat surface. As you aim your device's camera at the the template, tanks and planes start zooming around your virtual world. The template becomes a 3-D tower, so you don't actually see it on the device screen.
Aim your tower's cannons by tapping on the screen and shoot with a button in the lower-right corner of the screen. Other weapon controls and information are located at the periphery.
I have terrible coordination and have not managed to get past the third level, but most people will be able to shoot their way much higher. The fun is, of course, seeing these attackers interact with your real world environment and blasting them away before they manage to reach, for example, your coffee mug.
There's not a whole lot more to ARDefender, but it's fun and well worth the one dollar investment. It's not a terribly deep game but it's a fun one.
 Versions exist for the iPhone 4 (also the iPod touch 4th gen) and the Samsung Bada. Symbian S60 and Nintendo DSi iterations are forthcoming.