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.
Best Answer
I've been in the same trouble and in the version I played (PS2 & PS3), what you see in the video posted by Keaanu did not happen. In my version, there is a switch in the second plataform that start a machine below the second window. What you need to do in this case is to climb the piston of the machine and jump as high as possible. Take practice and several times but it can be done.
Let me tell you something: you are really close to the best kept secret of the castle! While you are on the other side of that window, try to hit the trees with the sword. What can be done with that? Is up to you to figure out! And let me tell you is really difficult (at least for me) to figure out without a walkthrough but you can use it in the very same room you are now.