In my opinion, this game has been remarkably under-valued by the PS2 community at large. This game never really seemed to take off very well on the market, and i can only hope its younger PSP brother is doing better.
The object of the game is stealth. As is mentioned before, the storyline is rather loose. It mainly compirises of "Choose a feudal lord or 3, find some purple rocks and defeat a sorceror". But, don't let the lack of storyline put you off. This game is superb. With 2 playable chracters, with their own array of stealth attacks between them, you'll have hours of fun sneaking along ledges, dragging unwitting enemies to their doom, and other similar styled attacks.
The landscapes themselves are tremendously fun. From forests, canyons and mountain passes, to towns and villages, and inevitably to mighty fortresses, you will have distract, sneak and kill your way through countless missions.
You even have your own fully customisable garden, with countless features to add to defend it. And defend it you shall, as the garden is often attacked by hordes of Barbarians.
The graphics may not seem as up to scratch as other games out there, but hey, its PS2, and can be forgiven for that, what with its more powerful brother, PS3, having all the good looks of the family. But seriously folks, don't let the graphics put you off. They aren't glitchy, and the movements are quite samey between enemies, but you still must be cunning to acheive you goal through many different mission styles, from total annhilation, assassination, theft, transport, kidnap, defense and rescue. This game can keep you playing til the end and beyond.