Gem Keeper is a fun iPhone tower defense game I have been enjoying lately. I picked it up for free on special as I don't tend to buy many iPhone games. The game starts simple and then adds more towers gradually in the first single player set of levels.

Ive enjoyed the tower that slows enemies down and the bomb tower. I am playing the game on easy and find that sufficent diffculty to be interesting. Ive just unlocked endless mode which is pretty diffcult on any level because you are going to lose, its just a matter of how long you hold out.