Defender Chronicles - Legend of The Desert King
With each update, this game gets better and better. Now with in-app purchases of gold and some items to help you get going, it's gotten even better. Each stage you have a different tower defense setup, and in the beginning, I didn't think the "hero" (there are two, General and Melwen) made any difference. Now, I see it very differently. Having played this game a LOT, it's pretty much my current top game, and I've come back to it time and again, it's replay-ability is very high.
It has RPG and tower defense elements, which are my two favorite genres and combined well, they are made for my ongoing enjoyment.