Walkthrough
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The Garden
There is a passage leading out of this courtyard, but for the moment it's blocked by a gate. Instead, jump up and grab the chain in the center of the courtyard, and start climbing up. On the way up spirits will attack, so you will have to slide back down into the courtyard to fight them, but it isn't too tough a fight. Once you have defeated all the spirits, climb up the chain again. When you get to the top, follow the walkway past the Idol Doors down into the garden.
Running along the center of the garden is a very slightly raised area with a number of grates in it. One of them it missing the bars, drop through this one into the passage below. Follow the passage towards the screen into some water. There are pressure pads up the small stairs to the left and right. Stand on the one on the right to open the gate in the courtyard, and call Yorda through. When she reaches you, get her to stand on the other pressure pad, opening the gate at the far end of this room. Go through the gate back outside.
Go to the far side of the pool and to Ico's left a little to find a pipe going up to the upper area. Run into it to grab it, and climb to the platform at the top. From there jump over the pool back to the garden proper. Go back to the unbarred grate, and help Yorda to climb out through it. When she is out lead her to the Idol Doors, and leave the area.