Instructions
Climb up to the ledge with Farah near the end of the "Above the Baths" level. This is the level where you get a new sword. The ledge comes after you find the second Sand Cloud and after Farah climbs through the crack in the wall and opens the gate for you.
Continue until you reach the room with the crank in the middle of it. This is just past the ledge you climbed up on.
Turn the crank clockwise until you hear it click twice. All visible doors to the room should now be closed.
Strike the wall opposite the door you entered with your sword. Continue striking until you locate an area of the wall that does not spark when your sword hits it.
Hit that area of the wall a few times to knock through it and unlock "Prince of Persia 2."