Despite the enormous number of Tower Defense games on the iPhone, we've managed to keep the number on this list to a bare minimum. Honestly though, in any roundup of great iPhone games, you can't ignore the significance of Fieldrunners. While not the original TD game by any stretch of the imagination, it influenced many that came after and is still the absolute classic on the platform. Fieldrunners is of the "open field" discipline where players steer the bad guys by creating a maze with (six different kinds of) gun emplacements.
Guide planes onto a runway by drawing the flight path they need to take. As time goes on, there are more and more planes that travel at different speeds. If two planes collide, it's game over. Another classic on the platform, and it has since been imitated in many different ways with many different subjects. The original is still the best though, plus it's cheap.
Yup, the Geometry Wars. It uses a clever contextual touchscreen thumbstick control system that tracks the movement of your thumb in relation to where you initially touch the screen. This means you aren't constantly fumbling to hold your iPhone in the right place. It's an elegant solution for such a fast-paced game. Touched includes familiar Geo Wars modes; Deadline, King, Pacifism, Waves, and Sequence.
PopCap does a sterling job with all of their releases on iPhone, and frankly you'd be well served by any of them, but Plants vs. Zombies is our choice for this list. This is the full-on PvZ experience fully optimized for a touchscreen interface; 50 levels, 49 different plants, and 26 different kinds of zombies. If you've not played it on the PC or the Mac, you're in for a treat. Think of it as a streamlined Tower Defense style game, only it's set in your back garden and it has zombies in it. An absolute classic.
Coming from the Space Invaders Extreme school of psychedelic game design, Infinity Gene is a hip, fast-paced variant on the classic shooter that blends traditional retro graphics with more organic, swarming invader behavior. As with Extreme, the game's visuals are tightly integrated with its thumping soundtrack, but Infinity Gene goes a step further by offering a custom level-generating mode that will interpret music from your iTunes playlists as custom firefights.