Touch Arcade: I adore tower defense titles, but they tend to be just a bit samey. More than most genres, they rely on a number of standard elements that are rarely done without. Kingdom Rush [$2.99] isn't a mold-breaking game. There are towers, yes, and upgrades and creeps. There is a set path, there are levels. Sometimes the creeps fly. Sometimes there are bosses. You really ought to know all this by now.