Walkthrough
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The Water Tower
Once you have saved, exit the passage and look left. The walkway carries on to the left, but there is a gap in it. Jump over, and then help Yorda do the same. A little further along the walkway ends. There is a bridge leading on further, but it is raised, so you can't continue that way. The rail on one side of the walkway ends before the walkway does. Over the edge at that point you will see a lower section of the scaffolding far below you, and beams on the side of the walkway which will let you climb down there. Slowly walk over the edge to grab on, and then drop down from ledge to ledge to the lower part of the scaffolding. Down here there is a horizontal pole leading over the edge of this platform. Stand under the pole and jump up and grab it. Shimmy along it until you get all the way to the far end. When you are at the end let go to drop onto a narrow walkway.
Go to the wall to the left and start climbing up it using the ledges on it as you have done before. At the top follow the walkway around onto the main body of the scaffolding. You can go two ways, straight on or to the right. The bridge you must lower is to the right, but you have no direct way of lowering it, so go straight on. A little way along there is a broken ladder on the right leading down. Climb onto it, and then jump off to land on a platform behind you. You must climb the tower that is next to you, on the side nearest you there are planks that will serve as ledges, so climb up them like before. When you reach the top, jump across the gap you just climbed up through to the platform behind you. Climb down the chain, turn to face the bridge, swing back and forth a bit, and jump at it. You should hit it, knocking it down into place. However, it does not completely bridge the gap, so you will have to go down and get Yorda to jump over to you.
Once Yorda jumps across lead her along the walkway and then to Ico's right, past the broken ladder, to where you have to drop down to a lower walkway. You can leave Yorda here as you do this next bit, though she may decide to follow you. You have to get to the area over in the distance on the left, where the Idol Doors are, but there isn't a direct way over there. Near where you are now, however, there is a water tower that has not stood the test of time as well as the rest of this scaffolding.
Drop down onto the lower walkway and follow it to a stone room. Inside swap your sword for a stick and grab one of the bombs. Carry it out to the far end of the lower walkway, where the higher walkway ends. Put the bomb down, light the stick using the torch in the corner of the wider area, light the bomb, and throw it in the general direction of the water tower. You have to throw it so that it doesn't fall short, or hit the tower and fall over the side. If you miss get another bomb and try again. When you succeed in hitting the tower, it will collapse, knocking down one of the higher walkways, and giving you a way to get closer to your destination. Go back and retrieve your sword, then take Yorda across the new bridge.
On the far side of the bridge is a nice solid-looking stone courtyard, but unfortunately the only way onward after it is a chain leading up to a higher area, so Yorda can't follow. However there is a crane mechanism above the gap between here and the Idol Doors. Climb up the chain to the higher area, and follow the wooden walkway out to the right. When it branches off to the left and right, go to Ico's left and push the crate at the edge of the walkway over the edge. Drop down onto the crate and push it over to the right, up against the wall at the edge of this area. Climb up onto it and then up onto the platform above. Pull the lever over on the left to move a chain out to the walkway you were on.
Drop back down to the lower area and go to the far end. Climb up the ladder there to get back to the walkway you dropped down from. Go to the far end of the walkway, and note that the chain is now 'bridging' the gap between the walkway and a platform with large lever on top. The first bit of this is the easy bit: jump out and grab onto the chain. Now comes the bit that is easy to mess up, jumping to the far side. This short chain doesn't have much give, so jumping from the right height is important. You should have landed on the chain with your feet about level with the ground on either side. Jump from this point and you should make it across. Assuming you make it, turn the lever all the way around to move the crane. Now get back to the walkway the same way you came. Go back to the far end of the walkway and climb back down the ladder to the area with the Idol Doors.
Jump out onto the giant crate and then over to where Yorda is. The spirits will finally launch an attack now, so you will have to deal with them before continuing. Once the portal is closed, jump back over onto the crate, and help Yorda to follow you, then jump over to the Idol Doors, and help Yorda over there. Get her to open them and go through.
Once again you are above the Main Courtyard on a high walkway. Follow it outside to another parapet and another bridge, leading to the second of the two reflector buildings. Once again, once you get there, Yorda will run inside ahead of you. The difference with this building is that it is a mirror image of the other one, so the layout is reversed. And, of course, there are different obstacles to overcome.