Reign of Swords is an amazing turn based strategy game for the iPhone. You're in control of your own army in a fantasy world full of opposing forces that want to take over your kingdom. Build up your perfect army and equip them with the best items you've received from your past victories. Then take the battle online against your friends.
If you've ever played Final Fantasy Tactics or Dofus, you'll be familiar with the combat system. Basically you place your troops on the field in a designated starting area. You have a set number of points you can spend on troops, allowing you to have a ton of smaller troops, a few larger troops, or somewhere in between. This makes every game a little bit different as you can change your strategy to suit your play style.
The single player campaign is very deep and will give you a great feel for how to play through. There's even a tutorial mode that nearly levels the playing ground for all of those new to this style of game. What's really interesting is that friends who don't have an iPhone can even play by going through the website and creating an account that way. Multiplayer battles seem to be done automatically, so you're not waiting for you friend to respond. It's more that their profile is being controlled by the computer against you.
 Reign of Swords is another must have game for the iPhone. It really does have every element that an iPhone game needs to be successful. This is one of the best strategy games to hit the iPhone since Galcon (and that was just this past week). Definitely pick this title up and expect to keep it on your iPhone for a long time.