Tower defense games are perfectly suited for the iPhone and iPad as they fall somewhere right in between turn-based and real-time strategy games. If you're unfamiliar, TD games are all about bad guys (usually called creeps) trying to get from one end of the screen to the other as you set up towers along the way in hopes of blowing them to bits before they get to their destination. There is usually time between waves so you can plan your defenses just right while not having to worry about obscuring the on screen action with your fingers.
NO.1 ► Epic War TD ◄
"If you’re looking for a more serious tower defense game that doesn’t include cowboys & indians or anything remotely cutesy, then Epic War TD is your game" - IntoMobile
"Epic War offers up some of the most action for the tower defense genre" - AppAdvice
"Epic War is one of, if not the best tower defense titles" - iFanZine
"Given the fact that each time you play the game randomly generates your enemies, it’s a game that can certainly have unlimited replay value." - IntoMobile