Let's be honest about something: tower defense games on the iPhone are a dime a dozen these days. The big problem is that, by now, there are a ton of really good entries in the genre, which means that it's increasingly difficult to create a tower defense game that actually stands out from the pack. Thankfully, Chillingo's Defender Chronicles - Legend of the Desert King manages to not only stand out, but also maintain a sense of challenge while staying fun.
The story in the game is actually pretty epic: a fantasy kingdom is being invaded and it's up to the player to assume the role of a heroic general who happens to be the only person capable of defending the land. There's a rather detailed plot that unfolds via fully-voiced narrative sequences between levels; unfortunately, the quality of the character's voice-acting is fairly mixed. Thankfully, the plot isn't actually crucial to the gameplay, and you can easily skip through these sequences if you start getting bored.