Start from the Above the Baths save once you have your new sword and head down the stairwell that leads to a large cavernous area. Get through the area until you end up in a small room with Farah. There's a lever and gear in the center of the room; turn it clockwise until it clicks twice. Both doors in the room are now closed, but you have also opened a secret door that you cannot see. Go to the wall directly opposite the entrance and swing until you find a spot on the wall where your sword doesn't spark. Strike this area a couple times to reveal a hidden room. Enter to unlock the game, which is now available from the main menu.
At about 83% through the game you will find a room with eight doors. In this room you will have to find the correct series of doors in order to continue. To solve this puzzle, walk by each door and, if you hear the sound of trickling water, go through it. Repeat this until a little cut scene plays.