I've always thought the iPad was brilliant for tower defence games, such as Kingdom Rush. A new brilliant tower defence game has also just been released to make use of that large iPad screen.
Fieldrunners has been a very popular game ever since launching on iOS years ago. The devs released Fieldrunners 2 for iPhone not too long ago, which came with a lot better graphics and smoother gameplay.
Fieldrunners 2 has just been released on to the iPad and supports the retina display the latest iPad has. I've been playing it myself and the graphics are truly awesome. Also the frame rate is super smooth, it looks very fluid.
You of course get all the features you would from the iPhone version too. Another great feature they have now added is the ability for your progress to be synced with iCloud so you never lose all that time invested into getting so far in the game.