You take control in the Casket Chamber. Now you must find your way out. The door leading out, which is currently closed, is underneath the staircase over to the right. The lever to open it is at the top of said staircase. Go up the stairs and pull the lever at the top, then go back down and go through the door.
On the far side of the door is a small room between the chamber you were in and the main tower. You can get into the tower through the windows at the far side of the room, but they are far above you. Climb up onto the higher area of the room, then jump up and grab the nearby chain. Climb up until you are level with the nearest window, turning to face it if necessary, and then jump from the chain over to the window. You may go through it, or land a little short, it doesn't really matter, just go through.
You land inside the main tower. The way out is blocked by Idol Doors. Suspended far above you is a cage. There are two ladders in the room, the one on the right (as you enter) leads up above the Idol Doors (but isn't a way out). The other one, on the left, leads up to a spiral staircase, which leads to the top of the tower. Climb up the ladder on the left and follow the spiral up. At this point the disrepair of the fortress becomes more apparent, with a smaller staircase completely missing on the way up. However, there is a chain in its place. When you reach the chain, climb up it to the ledge above, and carry on up the spiral. When you near the top, Ico will spot someone in the cage.
Now you must find a way to lower the cage. There is a lever at the top of the spiral staircase which will lower it, but a large chunk of the spiral is missing, so the direct way up is not an option. There is a ring of windows along the top of the tower, currently above your head. The furthest one you can reach, right next to the gap in the spiral, is broken. Climb onto the ledge under it, and then climb through it. When you get outside, go over to the rightmost window, which is also broken, and climb back inside. This will put you back on the spiral, past the gap. Continue up to the top, and pull the lever to lower the cage.
Now you must follow the staircase back down, but because you are going down you can take a couple of shortcuts if you wish. You can simply jump over the gap nearest you, and you can also jump down or run over the edge where the chain is, just make sure you don't jump over the side of the spiral, it's a long way down. When you reach the bottom, climb back down the ladder to the ground.
You may notice that the cage stopped short of the ground. All it needs to go the rest of the way is a good jolt. Climb up the ladder across from the one you just climbed down. Follow the ledge at the top along to the end of the room, and then across to the Idol Doors. Stand on the highest part of the arch over the Idol Doors, and then jump over to the cage.
Now you must rescue Yorda from the attacking spirit. Grab the stick near Ico to help you. If you take too long, more spirits may appear. Once you have defeated the spirit (or spirits), take Yorda over to the Idol Doors. She will open them, much to Ico's surprise. Go through the arch to the hall on the other side. Climb up to the doorway at the far end, helping Yorda on the way, then go out through it.
Written by Crumplecorn