Crystal Defenders is one of the worst tower defence games ever made and should not be played by anyone.
If you're not familiar with tower defence games, they go a little something like this: monsters troop across the screen following a fixed path, and you as the player have to erect towers to shoot them down before they reach the exit.
Crystal Defenders replaces the towers with characters from the Final Fantasy Tactics games, and replaces the monsters with... well, monsters. It's a strictly by-the-numbers affair. If you've played a tower defence game before, you've played this one.
The catch is this: the very best tower defence titles are Flash games, and are completely free to play. Crystal Defenders costs money, it has graphics which would look awkward on a 16-bit console, and it's significantly simpler and shorter than even the most basic of its web-based competitors.
For your money, you get twelve maps (fully half of which are little more than palette swaps), six deployable units, no in-game help or tutorial system, no unlockables, no story or victory animations, and an endless loop of some of Final Fantasy's worst crimes against the musical world.
It's also blisteringly hard. With no kind of guidance or strategy advice, even tower defence veterans will have a tough time clearing 30 waves on each of the maps. The strengths and weaknesses of your units aren't completely clear. Working out which units deal physical damage requires luck, guesswork, and some knowledge of other Final Fantasy games. Debuffs on enemies aren't marked, making it tough to assess the effectiveness of indirect damage, and survival ultimately requires not just killing the enemy, but correctly calling where you'll kill them, in order to allow you to deploy money-gathering thieves. Luckily, the availability of the internet will allow you to completely trivialise the game by playing a perfect round straight off the bat.
Crystal Defenders is available on Live Arcade, Wiiware and iPhone, and I understand it's exactly the same kind of garbage on each platform. It's emblematic of Square-Enix's general contempt for the casual and downloadable market and I urge you to avoid it as though it were made out of babies.