Tower defense is a genre that's almost run its course, if you ask me. But Defenders of Ardania is still a really solid tower defense title for the iPad that does find some innovation left in the process of setting up towers and letting them take out the incoming bad guys. The twist here is that while you're building towers for defense, you're also purchasing your own invading forces, which are then sent to wander out towards your enemy's castle. In other words, it's a kind of reverse tower defense, where you both attack and defend against an enemy doing the same.
Unfortunately, that makes things a little complicated -- you still have a gold pool which you refresh by killing bad guys (as usual), but you need to use that pool to summon both towers and invading forces. And as if that's not enough, you also have various spells to buff up your troops, and you can also lay down traps on incoming troops to stop them in their tracks, all while trying to bring down the enemy base's health and keeping yours up.
Like I said, it's complicated. That may appeal to some players -- the graphics are 2D but very well-designed and functional, and while there's lots to do, the UI itself is pretty well designed. Fans of more straightforward tower defense will probably want to look elsewhere (I'll recommend the great Gem Keeper yet again here), and those on the fence can check out Defenders of Ardania's free version.
Those up for a challenge who want something a little different from the tower defense genre should definitely give this one a look. There's a full campaign mode, multiplayer battles with Game Center integration, and lots of complex tower defense/attack gameplay to be had for just $4.99.