Instructions
Defeat Theseus after you reach the Steeds of Time. He can be defeated by constantly rolling through his attacks, using your longer range to strike back and shooting him with arrows when he leaps out of your range. Press "Circle" to perform a fatality when prompted to finish off Theseus.
Go through the rooms in this area and open all the treasure chests, as these contain items that will be useful later in the game. Use the altar in this area to unlock the ability called "Chronos Rage." Defeat the enemies that appear after this, which shouldn't be a problem as you're given infinite mana for the battle.
Go to the platform between the Steeds. There will be four turnstiles in this area that must be activated in the proper order. Activate the green turnstile, then climb up the wall to the section that this green turnstile opened up. Put the "Horse Keeper's Key" in the open section to free the Harpy.
Activate the red lever, then run to the first steed on the right and climb the wooden wall. Use the "Horse Keeper's Key" in the section opened up from the red turnstile to move the Harpy again. Activate the blue turnstile and climb up the wooden wall near the first steed on the right.
Jump and grapple over to the Harpy, then use it to swing across near the steed on the far right. Climb up the platform and use the "Horse Keeper's Key" in the blue device in this area. Activate the yellow turnstile, then climb up the wall on the left.
Jump to the Harpy and swing from it to reach the steed near the yellow device. Use the "Horse Keeper's Key" again to activate the final device. Go back to the platform in the middle of the steeds.
Press "R1" on the giant middle switch to activate it. Go to the front of the platform and press "R1" to start an action sequence. Follow the instructions listed on the screen to fully activate the steeds and get the island moving.