Ico is a little underground gem produced form an internal Sony development team. This diamond in the rough was something of a sleeper hit, and it attained such status without resorting to gimmicks (i.e. the new DOA volleyball). The game doesn't bring anything especially revolutionary to the table in terms of gameplay, but it's the artistic value that really sets this title apart. If ever someone says that video games are not a form of art, pop in Ico to change their minds. The graphics are stunning, especially the way light is displayed. If nothing else, I say you should rent the game just to see it in action.
The gameplay in Ico is very simplistic. You must not only make your way through the various rooms of the castle, but you must guide and protect the princess as well. You are able to run, jump, and climb your way across the rooms, but the challenge is often finding a way for the princess to go. You can beckon her, and she will come running across the room towards you. Once she is close, you are able to grasp her hand, and guide her along. She is not as nimble as you, but she can climb small ledges and jump across gaps (when you are there to catch her). The puzzles that present themselves in traveling from room to room can become fairly complex, though they never seem nonsensical. The answers to advancing in the game are almost always right under your nose. Also, forget about leaving the princess behind, she has the ability to open all of the magically sealed doors throughout the castle, so you need her in order to escape.
It is obvious that the princess has a sort of ethereal nature. It almost seems as if you are protecting a creature of pure divinity. It becomes a very rewarding experience to guide and protect her. Stalking her are amazing-looking "shadow monsters" that come out from portals in the ground. They will ignore you (unless you attack them), but they will try to grab the princess and drag her inside of these portals. You must fend these creatures off, using only a light wooden staff which you pick up in the beginning of the game. Using simple attacks, you must either batter them until they are vaporized, or you must run across the room with the princess, evading them along the way. These encounters are very well-done, and can become quite fierce when there are 50 or so of the monsters on the screen. You even become distressed and alarmed when one of the monsters manages to knock you down, and drags your female companion towards an ominous black portal. The characters are very emotive, and easy to relate to.
Overall, the game can get a bit repetitive, and is on the short side, so you may want to just rent it. However, it's one of those games that I'm proud to have in my library.