Gem Keeper is a relatively new game from developer NCSoft, which usually makes Korean MMO titles for the PC. The company has released a couple of apps for iOS, but this is the first full game made for North American audiences -- and it's a good one. It's a pretty basic tower defense title, but the polish and complexity on this one really makes it shine, even among the many offerings in the genre.
As you can see from the screenshot, the look is great -- it's bright and colorful, and very easy to tell just what's happening on screen and where you need to step up your defenses. The game's balance is very good as well. I would say that it's a little tougher than a lot of tower defense games, but only because not only do foes have to reach your stash of gems and steal them, but they have to wander back out as well, which means that the stakes are upped twice as high as games where you're just trying to prevent an invasion. Gem Keeper also offers multiple places for waves to come through, which adds a little more strategy to how you set things up, and extra units like moveable towers and bombs mix things up even more. The game's currency flows fast and easy, especially in the lower levels of difficulty, so you always feel powerful, even when you're being flooded by invaders.
There is plenty of content to play through -- there are over 30 levels with three difficulty variants each, and a recent update added an endless mode which you can compete in on Game Center leaderboards as well. Gem Keeper is a great game -- it's on the App Store as a universal version right now for $2.99. Here's hoping we see some more quality titles like this from NCSoft in the future.