There are tons of tower defense games on the App Store, and for good reason-- The touch interface and fairly slow-paced gameplay just works fantastic on iOS devices. I have quite a few favorites in the genre, but Origin8's Sentinel 2: Earth Defense [Free] is always on the top of any list of tower defense games I recommend. There is just so much I like about this game which I explained in my review.
In a nutshell, Sentinel 2 has great graphics, good sound effects, a nice variety in both towers and enemies, and the inclusion of both repair drones as well as different destructible barriers to defend really adds an interesting layer of strategy to the game. There are also mega weapons that are fired from your orbiting space ship that slowly recharge over time, my personal favorite being a massive orbital laser strike that destroys anything it comes in contact with. The best thing about Sentinel 2, at least in my opinion, is how well the game is balanced.
There doesn't seem to be any "right" way to beat the included levels, as quite a wide variety of different tower constructing and upgrading strategies all seem to work. This makes the game a lot more fun than a lot of other tower defenders which often seem to be games of how intelligently you can manage your resources to ramp up to whatever the fully upgraded ultimate tower is that kills everything in one hit-- Allowing you to just put your phone down and wait for all the waves to just die to your defenses and for the next level to start.
If you even have a vague interest in tower defense games, or even have never played one before, you should download Sentinel 2 while it's free. If you miss the window of the freebie promotion, I'd even recommend buying it.