I've expressed many times before how good Kingdom Rush was on both the iPad and iPhone. The developers charge for the app and so they should, the amount of work that goes into producing something like this is amazing. What is even more amazing though is that the game is available for free on the iPhone. It's available for a limited time only so download it ASAP. If you like your strategy games, you'll absolutely love this one - the graphics highly remind of the Pokemon series on Gameboy.
Also on a side note - there is now a Chrome version of the game, but it actually links to their web version of the game, still, it's awesome - Kingdom Rush Chrome