There's a chain hanging from near the center of this area below the plaza. Have Ico jump onto the chain and begin to climb up. As he does so, the next round of shadow demons will spring to the attack. There are far fewer than the last several battles this time and he should be able to defeat them without too much trouble. Once done, it's time to again climb the chain, but this time all the way to the top. Ensure that he's facing the ledge, that the chain's hanging from as he nears the top. Walk forward and note the closed Idol Doors. Turn to the right and walk down the stairs and explore the area a bit. You'll note that one of the storm grates is missing and appears to be the only obvious exit. Descend and drop into the corridor below.

Head down into the partially water filled area. Note the two pressure switches with stairs leading up to them out of the water. Have Ico stand on the right pressure switch, then call to Yorda to come. Once she's arrived, move over to the left pressure switch and call her again. Give her a hand up onto the platform and lead her onto the pressure switch. Depart and go through the now opened rear gate. Have Ico climb up the pipe right in front of him, then turn left and move past the end of the large pipe so that he's standing in line with it. Turn and jump across the gap. He'll just make it if he doesn't jump into the water, which he'll probably do the first time or two. Go to the open storm drain, stand on the right side, and call to Yorda. When she arrives, pull her up Take her by the hand then lead her up the stairs to the Idol Door and on through it.