If you have a PSP and a PS3 you should check out this demo... at least, if you like either tower defense games or simulation games.
Basically you play a dungeon lord and manage your dungeons monsters to keep out pesky heroes.
The game itself is very short, but is very deep and repayable. The main mode feels like an arcade game... might only take 5 minutes per play, but you will want to play a lot. There are also a ton of specific challenges that teach you how to manipulate the ecosystem of monsters, and a second story campaign that is much tougher. I love this game, but its definitely try before you buy...
Trailer of the game