Yorda has no part to play until the end of this next section, and I won't mention her, but make sure to keep her in the same area as you at all times, despite the fact that you're moving between the same few areas. Go through the doorway to the left into a large hall.
This hall has two floors, and each floor has an exit at either end, one going to the platform you just came from, one going towards the front of the fortress. You enter the hall on the upper floor, and can go out through the door leading to the front of the fortress on this floor, but there are only raised drawbridges on the far side, so it is of no use. You can also get to the lower floor via the platform you came from, but the lower floor also features a large chasm right in the middle, so you can't get to the far side. There is a chain providing access to the far side of the lower floor from up here, but that is no good to Yorda. The one good thing is that the bridge traversing the upper floor of the hall is damaged. What you must do is knock this bridge down, so that it will lead from the top of this side to the bottom of the far side.
Start by going to the far end of the hall, to end with all the small windows, and more importantly all the window ledges. Climb up the ledges, the way you climbed the ledges outside the crane control room, until you get to the top. When you get up to the top, climb up onto the nearest wooden beam. Walk along the beam to the chandelier in the center of the hall. When you get there, drop down onto the chandelier and hit the chain holding it up with your stick. The chandelier will fall, damaging the bridge. Most of the chandelier will fall down into the chasm, but a section of it remains on the lower floor. You will end up back on the bridge.
Go back outside to the circular platform and find the spiral staircase across from the doorway. Go down the staircase to the lower doorway. Grab a bomb from just outside the doorway, and then go in. You are now on the lower floor of the hall. Put the bomb next to the support pillar of the bridge, then go light your stick using one of the candles on the chandelier fragment, then go and light the bomb. The blast is enough to collapse the bridge, making a direct route from the near side upper floor down to the far side of the lower floor.
Go back the way you came to the upper floor of the hall. Now you must go down the bridge to the Idol Doors, but when you start down a large number of Sentinels will launch an attack. The easiest way out of this one is to drag Yorda quickly down the bridge to the Idol Doors. Once she has opened the doors, go through to the Main Courtyard.
Here the spirits launch another large attack. Again the easiest way to destroy them is to get to the Idol Doors, especially since the portals are very widely spread this time. However, it is not a straight line to the Doors this time. They are actually directly across from where you are now, but the route there is circuitous. To get to them, go down the wide staircase in front of you to a narrow walkway. Go to Ico's right and follow the walkway to a narrower staircase. Go down this staircase, then go along the courtyard to Ico's right. The Idol Doors are in the center of the courtyard, up another staircase on Ico's right. If you do get stopped by the spirits on the way, be careful, as there are several waves in the attack, so as soon as you get Yorda back, more will start on you again.
When you get to the Idol Doors and Yorda opens them, she runs straight through. Follow her.
Written by Crumplecorn