I thought I just about seen every kind of tower defense game out there - until, of course, Star Defense. The mechanics are pretty much the same as any other tower defense title, however what makes this unique is that levels are viewed as full 3D planets, with a Super Mario Galaxy twist.
Star Defense comes from ngmoco - the same guys who brought us LocoRoco-clone Rolando. The game has multi-touch support, with seven full levels, various upgradeable towers, and three difficulty settings "upon launch" (which hints at possible DLC in the future). There are also medals and achievements to plot your progress. The game is due on the App Store on early May.
Looks like Square Enix's Crystal Defenders will be getting a run for its money. Check out these screens and teaser trailer. Amazing.