The Old Bridge
In this room is your first puzzle. However, before you can figure it out you have some spirits to take care of. When Yorda goes down the staircase in the center of the room they will attack, a Sentry and some Spider Wraiths. The spiders don't pose an immediate threat, so attack the sentry first. When they are all defeated and the portals close, you can start looking for a way out of here. The door at the bottom of the stairs is a way out, and you can go out through it, but the bridge beyond is lowered, and can only be raised from the far side, so you'll have to find another way out. To both sides of the stairs in the center of the room are small pits. In one of them is a crate. Push it, in any direction, to reveal that it was on a pressure pad. Moving the crate makes a larger staircase in the room rise up. Go up this staircase and exit the room through the doorway at the top. Follow the walkway outside into another large hall.
You enter this hall on a walkway, and there is a closed door blocking the route down to the main part of the hall. There is no way forward for Yorda for the moment, but there is for Ico. Just next to the doorway you entered through there is a gap in the railing, and below this gap is a pile of crates. Leave Yorda where she is, and drop down from the walkway onto the crates. Now you're in the main part of the hall.
Up along the hall are a small staircase, a Stone Seat, and the blocked way up to where Yorda is. Across the hall is another walkway which is where you need to go, as the lever that opens the door is up there. Go across the hall to the other walkway. There is no stairs here, just a chain. Climb up it, and jump from it to the walkway. Pull the lever to open the door, and then push the nearby crate into the lower area. Sooner or later spirits will attack, so drop back down into the lower area to defend Yorda. There are Sentries and Spiders again, and also Warrior Spirits. Once you have destroyed them all and the portals have closed, save if you wish, and then help Yorda up onto the walkway. This involves getting up there yourself and then helping her to climb up there from on top of the crate. If you go up there and lean down over the crate she will climb up onto the crate and then up to you. Once you are both up, go through the doorway to another outdoor walkway.
Written by Crumplecorn