TouchArcade: I discovered tower defense late in my gaming career. I have no good justification for this. Perhaps I was too busy burying my face into all the RPGs and sidescrollers I could find. However, my intro game into the genre was PixelJunk Monsters, and if you gotta discover late, it may as well be with one of the genre's best entries. Ever since, I salivate Pavlovian-style every time someone murmurs the words "tower defense," and I had the same reaction when Gem Keeper [$2.99] was mentioned around the Touch Arcade office. I also was digging the fact that NCSoft made it, because I like them and I wanted to see how their skills would translate to iOS.