Well, there's Final Fantasy XI, but.... I won't bring that up.
I played it for a little while on my brother's iPhone. It basically brings a barebones version of the Final Fantasy experience into the Tower Defense formula. You have to choose the right characters for the right situation, like using archers to take out airborne enemies and black mages to set targets that are farther away on fire. The only way to lose is by letting a certain number of enemies through your defenses. Otherwise, the game is pretty simple, and fun to play. I don't know if it was necessarily worth $10 since it is an acquired taste, but if you have always wanted to play Final Fantasy and all you have is an Xbox 360, and if you have points to burn, I say go ahead and give it a try.