Final Fantasy's Tower Defense spinoff
There are 3 types of maps, W1, W2, W3. Each in increasing difficulty, W1 being easiest and W3 being the hardest. By using your units, you have to withstand the hordes of enemies. You are given 20 crystals, each enemy that passes through will deduct one crystal. In some cases, two. There are 30 waves in each map, progressively getting harder each time. It's a difficult game, depends on how good you are at this game.
On a side note, you can load a custom soundtrack, which isn't too bad because it can take almost an hour of your time for each round/game. All you have to do is press the PS Button then go to music in the xmb and just play it. Unless if you like the game soundtrack then you can keep it. It's a bit frustrating to go far then lose... which is what happens to me, but it's still a pretty fun game.
I'm not sure if it's worth 10 dollars though... especially when the PSP version which is the exact same is cheaper. Maybe a 7.5 is a bit too generous, but depends on you.