The Hall of Demetrius is a large open space with balconies and paintings. It looks rather grand but the level itself is just an antechamber between the Lost Library and the Temple of Pharos. You start in the hallway beyond the double doors of the Lost Library - follw this to the end and enter the main room.

Go up the ramp on the right side of the room and round several corners until you enter a small room at the top. Take the Pharos knot from the pedestal. Return down the ramp to the main room and cross over to the ramp on the opposite side. Climb this to the room at the top.

In a cut scene, Lara picks up one of the scrolls on the floor and begins reading. She is interrupted...

Von Croy has made good his escape and Lara finds herself on the floor of the main room with three assassins between her and Von Croy's exit. Wait in the main room, facing the ramp, until the assassins appear and then blow them to smithereens to make yourself feel better! Now head up the ramp to the set of bookcases that Von Croy escaped through.

Nearby there is a floor standing lantern with some scrape marks indicating it moves. Drag it along the scrape marks to open the secret bookcase panel. Go through the new exit and follow the corridor to a pit. Jump into the pit and slide down the ramp to splash into a small pool back in the Coastal Ruins level. This is Secret #1. Climb out of the pool and pick up a large medikit and poison arrows from two corners of the room.

Explore the shrine in the middle of the room and pick up the broken glasses that, presumably, Von Croy dropped when he made the same trip. Look up into the roof space of the shrine to see your way out... Climb the ladder up the inside of the shrine's roof and follow the passage at the top to emerge at one of the building overlooking the sea in the Coastal Ruins level.