Gravity Defender is an iPhone game whose gameplay is based solely on accelerometer input. The player owns a small, triangular space ship in the middle of the screen,
but has no actual control over where it moves, where it aims, or when it shoots. Instead, the player simply rotates and tilts the iPhone to affect the trajectory of
the bullets fired from the ship. The "real world" gravity detected by the iPhone's accelerometer is applied to the in-game bullets.
I consider the design of the game to be very successful, though in its current state the difficulty curve is too steep. I put a lot of work into making the game very
easy to start playing without any instructions. A key part of the introductory stages is a "training line" which shows the predicted trajectory of the bullets. This line
gradually fades away as the player advances. A big lesson learned from playtesting was that as more elements were added to the game, it often became less fun.
For example, multiple player ships made the game more challenging, but having them rotate in opposite directions made it too hard to control.
Nearly everyone who played it was instantly hooked and didn't stop playing it until they either finished the 15 stages or got stuck and gave up out of frustration.