Every map has a series of roads, usually interconnected with one another. Enemies will manuever along these routes to get from point A to point B. To counter that, you spend Gil to buy characters (Soldier, Archer, Black Mage, etc.) and place them at strategic points on the map alongside the road. They each come with an attack rating and a line of sight/range. When enemies get into their range, which is indicated with a circle around the highlighted characters, they move and attack. When all the enemies are killed, the round is won. You earn more Gil and Crystals in order to spend on more characters to place and expand your defenses, or to improve the range and attack of your pre-placed characters. Of course, don't worry if you screw up and forget to place some new troops down and start the round. All you have to do is hit X and then you can place a new character down. Simple, right? Of course, this same game's been done before. Again, Ninjatown for the DS.
It's not the prettiest game on the PSN (and yes its also on Xbox Live, if anyone wants to correct me), since it still uses FFTA2's graphics engine.
The game is very simple and the only way it challenges you is with more advanced maps, stronger enemies in the later stages, and increasing costs of leveling your allies making it more challenging to defend against later waves.
The game also has trophies/achievements enabled, and every map has Leaderboard scoring set up to challenge players with comparing their high scores to one another around the world.
The price point is also reasonable, at 9.99.
I thought the game was quite enjoyable since it's the kind of game that you don't want to invest too much thought into, especially after a long day, and it gets surprisingly addictive.