I first played Kingdom Rush as a Flash game online, and it's very good in that context as well. It's pretty straightforward tower defense, in that you face a series of invading monsters, and need to build up towers to defend against them. But Kingdom Rush outgrows its simple tower defense roots with a number of intriguing innovations. Instead of just building towers, some towers provide you with actual units to control, and you can set rally points for them and line up your attack a little more carefully than just placing and forgetting. Kingdom Rush also makes heavy use of spells to mix the action up, and there's such a solid balance between spells and towers that more often than not, it's your spells that win the day.
An actual talent upgrade tree helps hit that note a little harder: As you play, you earn talent points that can strengthen a certain part of your game, such as beefing up your ranged towers, or allowing your spells to be stronger or earn more gold. By tweaking that talent tree, you can play Kingdom Rush very differently from someone using different talents, and the excellent polish and production on the game (I would argue the iPad version looks even better than the Flash game) means it's lots of fun throughout the many various levels.
Kingdom Rush is highly recommended, both as a tower defense title, and even as a strategy title for someone who might not have found a TD game they enjoy. The game is $2.99 on the App Store right now, and updates are planned, to add both Game Center integration (for the game's many achievements) and new content and enemies. Excellent title, definitely pick it up for the iPad if you haven't yet.