Walkthrough
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The Main Courtyard
You will be attacked by Sentinels again once you get outside, destroy them as you make your way along the bridge and parapet. When you get to the far end of the parapet, before you go through the doorway back into the Main Courtyard, climb up the ladder to the right of the doorway. This is a very long ladder, and you'll be coming straight back down, so as long as you don't stand around looking at scenery at the top, you should leave Yorda at the bottom. Climb the ladder and go through the doorway at the top. It leads to a platform far above the Main Courtyard. Run over to the right and pull the lever (which lowers a drawbridge), and then go back down to Yorda the way you came without delay.
Go through the doorway next to the ladder into the Main Courtyard. Go across the drawbridge you just lowered, and follow the walkway on the far side all the way over to the left. At the end you should find a crate. Push/pull it right until it is next to the nearest pillar. Climb up onto the crate, and then up onto the pillar. Jump over towards the rope to the right, and swing your sword at the same time to cut it. This will lower a rope into the lowest part of the Main Courtyard.
Go to the middle of the walkway you're on and through the doorway. This is the hall that had that chandelier. Go to the far end and down the bridge and out to the Main Courtyard as you did the first time through here. Once again you will be attacked along the way, but this time the Idol Doors are already open, so you can stay and fight, or run for the courtyard. Either way, once you get out to the courtyard, go down the first staircase, follow the walkway off to Ico's right, and when you go down a narrow stairs go through the arch that is almost directly across from it. Go past the graves on the other side to a wide open area. Go to the rope at one end of this area, and jump up and climb onto it.
Start climbing up the rope, and note the broken window across from you. Ignore that one, and climb up to the window one floor up, which is also broken. When you are level with it, jump over to it. Drop onto the narrow walkway just inside the window, and follow it into the room over on the right. Push the crate in this room into the room below, and drop down after it. Move the crate over to the broken window in this room, and then push it through the window, down into the courtyard. Help Yorda to climb into this room using the crate. Take her up the stairs across from the broken window, get her to open the Idol Doors, and go through.