After you put in the code to open the gate by the waterfall (2126), make sure that the guard is dead. Go right from the open gate. Go along the stone wall/fencing and gate to the end of the walkway. On the wall, you will see a faint line between darkness and lightness. Go to the other side of this line and walk forward. You will pass through the wall into a small area in between two larger walls. From here, you might be able to do a split jump.