You could beat Kingdom Rush, one of the best tower-defense games for iOS, and still never really finish it. The gameplay was so exquisitely balanced, with maps that remained challenging after dozens of plays, that there were always good reasons to come back to it. Now we have a new reason to return: Kingdom Rush Frontiers, the tremendous sequel from Ironhide Studios. Tasked once again with defending the perpetually endangered land of Linirea, you’ll find a range of new tools at your disposal. The game’s four towers have been redesigned and given different abilities, and a fresh crop of "heroes" (available mostly as in-app purchases) can augment your defense.

As with the original, the genius of Kingdom Rush Frontiers lies in the way it regularly puts you on the brink of disaster. Some of the game’s 40 new enemies go temporarily invisible, rendering your attacks ineffective; others speed up as they are injured; one nasty beast kills your soldiers with a single shot. Beating them requires careful planning both in the way you lay out your towers and the order in which you upgrade them. You’re unlikely to get it right the first, second, or even third time, making Kingdom Rush Frontiers that much more replayable. The rush of the game’s title comes from the moment you finally find a strategy that works, fighting off your foes and living to battle another wave. Keep ‘em coming.

Kingdom Rush Frontiers is available for $2.99 on iPhone and iPod touch and $4.99 for iPad.