Walk straight ahead, then turn to the right, and continue to the ladder, that Ico came down. Have him climb the ladder, then call to Yorda to have her join him at the top. Next walk to the right and traverse the next ladder in the same manner. Once they're both on the top, have them stand on the pressure plate and join hands, then walk through the now opened door. Walk across the pathway to the vault and pass through the doorway. Next head down either pair of stairs. Head to the ladder and climb up to the small platform, move about three steps to the left, then jump and grab the metal rail above. Head to the left. Drop off onto platforms to switch rails as necessary until Ico reaches their end. The solution for the European version differs. There's a gap in the metal rail leaving Ico no way to cross over. However there's a switch on the third platform near the gap. Drop onto the platform and throw the switch. Have Ico descend on the nearby ladder until he reaches its end, then drop to the floor. Head over to the machine, that the switch activated, and stand on the wooden pole, which will bend a bit and then fling ICO up into the air. However, unaided it will not get him far enough. Timing as usual is important here. Have Ico jump just as the pole begins to go back down and he'll sail high enough to grab onto the stone ledge. Press the 'Triangle' button a few more times and he'll be up at the window in no time. (Thanks to Guðlaugur sigurjónsson for the foregoing.) Then head out the open window and climb down the ladder. Note the running water to the right and the turn style type control above it, that can be used to shut off the water. But that's for a bit later

Drop down off the end of the ladder and walk around the stairs to the left. Pull the switch next to the stairs there to open the door above the stairs. Have Ico stand next to the nearby tree and give it a good whack with his sword. Doing so will cause an apparently heavy semi-round orb to fall from the tree, which nonetheless can be thrown a good distance and will bounce about when it lands. Have him pick it up and then head up the stairs and rejoin Yorda inside the vault. Look to the left (back towards the two sets of stairs) at the three doors in the wall and note the different colored one in the center. Pushing on only the right side of it will cause it to pivot on its axis and swing around so that they may pass through into an adjacent room with another of those slowly pulsating circles on the floor.

Stepping on the circle with Yorda will cause a fairly square basket to be raised on a pedestal on the stage in front. Ico then needs to get up on the stage and stand to either side and toss the round object into the stone basket. This is easier said than done! A simpler solution is to first go up on stage and place the round object in the square outline on the stage floor. Now take Yorda down to the pulsating circle and have them both stand on it. The round object will fly up and then drop back into the stone basket as it rises.

If you wish to do it the hard and honest way, Ico can throw the orb into the basket from several angles, but the one that should be the easiest is from the left. He needs to be two or three steps from the wall and running toward the basket when he throws it. You'll likely make a number attempts before being successful, Once done, a mace will fly through the window behind Ico and fall to the floor below the stage. Ico can now replace his sword with the more powerful mace.