Prime World: Defenders was released for the PC/Mac sometime in 2013 – last month, this card collecting tower defense (TD) game, now simply called Defenders was released for the iPad (you can also check out appSIZED's review of Man At Arms TD, which employs the same mechanic).
There is a story to this game and your mission mainly is to defend powerful and valuable prime (crystals) from the Touched (enemies or creeps, in TD parlance) who want to take it for themselves. Caught in the middle of two warring groups trying to control the resource, your job is to keep them away using various powerful towers.
Fieldrunners (iPhone) is a tower defense game that combines cute cartooony runners, vehicles and flying objects with serious looking towers that lob plasma, bullets and shoot electricity. Long before the tower defense (TD) genre spawned hundreds of copycats as much as it has some original concepts going, Fieldrunners for the iPhone was the definitive game of the tower defense genre and iOS app development. Tower defense games have multiplied exponentially since then, from cute themed games to hardcore action.
Meanwhile, Fieldrunners for iPad ($7.99) came out in 2011 – meaning indie developer Subatomic Studios took their sweet time to port the iPhone version to a bigger screen. Fieldrunners 2 HD ($7.99) on the other hand, launched to great success late this year, and is retina-display ready.