If you aren't sick of tower defense games by now, then GemCraft might be just what you need for your latest fix. Heck, even if you are sick of them, this might be the one to bring you back. It brings interesting design and gameplay to the old tower defense table.
It is actually a port of a monumentally popular flash game by the same name, now converted for the iPhone (much like Vector TD, among others). It vastly improves on the original by simplifying the menus and optimizing the user interface for a touch screen, creating an easy-to-pick-up-and-play experience.
The game has you, a wizard, place towers and various colored gems around a map to defend against the hordes of creepy creeps that want you dead for whatever reason. You can place towers off the path, or traps on the path itself, and then equip them with gems to create your network of defense. Every different-colored gem holds a special characteristic, such as slowing down enemies or poisoning them. Where this game really shines is in its gem combination system, which allows you to combine any gem with any other gem, regardless of color or level. This adds a level of customization and strategy that is not found in most TDs.
It also plays kind of like an RPG, in that you actually level up and unlock skills, such as reduced tower cost, which will carry over to any game you play. You also have to level up in order to unlock the different worlds and game modes. Normally that would be fine, but it takes too long to unlock everything, and I could only unlock three of the seven modes before I had to make myself stop to write this review. On top of that, the game can feel long or repetitive at times, even when fast forwarding rounds.
The game supports Game Center achievements and leader boards, giving you extra stuff to strive for. There is a free version to try before buying. Overall, it's a very refreshing experience that mixes up a tired old genre, and is definitely worth a look if you're a fan of these games.