Kingdom Rush is among the most addicting tower defense iOS games in the App Store, and it's currently available for free — down from a normal asking price of 99 cents. Be forewarned, however: this fantasy title is likely to consume you (and hours of time) as you work to defend your castle / tower from demons, trolls, orcs, wizards, and any number of other enemies — over 45 in all. Over 50 Game Center achievements await those willing to put in maximum effort. Still, though the game won't cost you anything (for a limited time), those in-app purchases can quickly add up, with some running as high as $9.99. But again, considering the core game is free, there's no good reason not to add this one to your "games" folder.