After finishing the waterfall area, you cross a bridge with a fence on either side, and enter a large open room with a ladder to the top. At the top of the ladder there is a pipe that goes most of the way around the room, but it stops short of the window that I assume I'm supposed to go through. I can't for the life of me figure out how to get to that window! I think I must be missing something dumb?
From the Walkthrough:
Go through to a small bridge. Cross it to a large shed with no way out save the way you entered.
Find the ladder going up. Climb it, leaving Yorda where she is. Shimmy across the pipeline to the outer window, then climb to the ground using the ladder. Push the block into the water, letting it flow down with the waterfall, then jump across the stone aqueduct and go to the ladder at left.
Climb it to a switch; push it until the water stops flowing, dammed by a wooden block.