Good to see another Ico fan around- this game was sorely overlooked and really should be checked out by everyone.
Starting from where you destroyed the water tower, follow the new path made from this and climb the chain to your left. Turn to the right when you get to the landing and you will see a box to shove over the edge to where the idol doors are.
Now, climb down the ladder located opposite the chain and push the box to the far wall to the right of the doors. Jump up onto the box and pull the lever to bring out a chain next to the beams above you. Head back up the ladder and jump to the chain. Swing to the other side to the wheel (if this is where you are falling you may need to adjust your height a bit on the chain). Once you get the wheel turned slide down the chain to Yorda and be ready to fend off some shadows (or just rush Yorda to the doors and avoid the battle).