BioDefense: Zombie Outbreak is a one-player zombie-survival game that mixes elements of real-time strategy with an open-map tower-defense game.
The core of BioDefense is a survival mode, in which you protect your base from random waves of zombies. You have to juggle several interdependent factors as you construct and defend your base: you have to build collectors on top of "toxium" deposits (toxium is the game's currency, and a finite resource on the map), but you can only build where you have light--so you also need to build spotlights. Predictably, spotlights are fragile, so you need to defend them and your other buildings with upgradable rocket launchers and machine guns.
You also need to build enough power generators to support everything you've built, and power cables automatically link nearby structures back to your headquarters. If zombies destroy spotlights, they leave nearby weapons helpless--and their attacks can likewise cut off satellite structures from your headquarters (and power) entirely. You can upgrade weapons, collectors, and generators, but first you have to spend toxium to boost your tech level. In addition to the survival mode, the game also has 10 missions with specific objectives and timers, which you can unlock by getting farther in survival mode.
BioDefense has many shortcomings--the art is lackluster (especially compared with other tower-defense titles), the gameplay can be repetitive, and the build and resource accrual rates never seem quite right--but its pioneering use of RTS-style features like fog of war and resource collection make it a remarkable addition to the iPhone and iPod Touch (and especially the iPad, with BioDefense HD). Fans of TD and RTS games should definitely check it out.