Walkthrough by Mulf
Go through the crawlpace and around a corner into a small room with a large vase (containing shotgun ammo and a scorpion). Go up the steps and get your shotgun ready (it’s in your inventory from the start), as a skeleton will rise out of the ground and will soon be joined by another one while you make your way to the climbing pole straight ahead (alternatively, ignore the skeletons and simply run and jump towards the pole). Slide down into the room below and use the cog switch to raise the door ahead (four times gives you ample time to jog through at your leisure). In the room behind it, climb the ladder until you reach the top of the pillars left and right. On top of a third pillar, in your back, is another large vase (containing shotgun ammo and no scorpion). Jump back towards the ladder and climb or jump directly to the top; pull up into the next room. This concludes the introduction, as you have now found the Lost Library.
Push open the double doors ahead. Two scorpions come towards you as you enter the hub room you’ll recognize from TR4. There are five doors on the ground level; one (S wall middle) can only be opened from the other side, one (N wall left) leads to a dead end, and one (N wall middle) stands invitingly open, but yields only one shotgun ammo pickup (climb the ladder to the upper story; none of the scrolls you find here are interactive).
Return to the hub room and open the next door (N wall right). This leads to an area in grey and bronze which looks like the old haunts of the tinman on horseback, but he’s not around anymore. Instead, you meet another skeleton; blast it into the spike pit straight ahead. On the E wall there is another large vase (containing a scorpion and no shotgun ammo). Go through the doorway next to it and turn right. Ahead is a closed door, with the mechanism that operates it next to it. Stand in front of the bronze ball and jump straight up with pistols drawn to shoot it.
When you hit it, the door opens. Behind it is another skeleton; to get rid of it, use the same spike pit as before, then return to the room the door of which you just opened and climb the pole in the SW corner until you can backflip onto a higher ledge (Lara should be about level with the fire bowl). Pry the purple Gemstone off the wall with your crowbar (which has been in your inventory from the start, too), then safety-drop down to the floor (losing a bit of health) or use the climbing pole again, and return to the larger room with the spike pit.
Locate the jumpswitch in its NW corner. Another skeleton is triggered when you step onto the block in front of it, so go back down and get rid of it in the accustomed manner (on this occasion Lara kept targeting the dead scorpion, now invisible, after loading a savegame). Return to the jumpswitch and use it. A rumbling noise announces a couple of blocks rising out of the ground in your back.
Climb up and after going up a few steps, turn left to shatter a wooden barrier, behind which is a small medipack. On entering the larger room ahead, you’ll encounter another scorpion. As before, all the scrolls strewn about are non-interactive, including the one placed eye-catchingly on a pedestal; so just pass through this room and an adjoining corridor until you arrive at a door leading to the balcony that runs along the walls of the hub room.
Turn left (E) and open the nearest pair of double doors to find yourself on the ledge opposite the one where you might have picked up some shotgun ammo earlier; continue to a small room with a Coin Piece (1/2) on a pedestal. An adjoining room, similarly small, holds nothing of interest.
Return to the hub room and run across to the S side of the balcony. The only door that opens here is the one in the middle. Follow the corridor behind it to another room that appears to hold nothing of interest.
But note the scratch marks on the floor, indicating an collapsible piece of floor on an adjoining tile. Stand on it, drop down and follow the short corridor to a dead end where you can pick up a big medipack and shotgun ammo. This is a secret (1/3). Go back to the balcony.
Since the other doors up here don’t open, jump back down to the ground level (losing a bit of health) and continue your quest with the doors on the S wall. There are only two here, and of those only the one on the left will open.
Enter and shoot another wooden barrier on your right for flares. As soon as you enter the main room ahead, two fire wraiths are triggered. Turn left (N) or run straight ahead (E) and follow either passage to the same room with a pool of water. Jump in and wait for the wraiths to extinguish themselves; then climb out of the pool and retrace your steps to the room in which they were triggered. Take note of the closed door there which requires an item to open. and shoot the two large vases (each containing shotgun ammo and no scorpions). Return to the pool room via the N passage and pick up a small medipack from behind another shootable wooden barrier.
In the pool room itself, move the large pushable vessel on its designated tile to open the door next to it. Follow the corridor until a skeleton jumps out from behind a corner. Blast it into the pool of water, then go back to pick up another Coin Piece (2/2) and shoot the two large vases (containing, respectively, shotgun ammo and nothing). Return to the room in which the fire wraiths were triggered, combine the two Coin Pieces to form an Owl Coin, and place that in the receptacle next to the closed door.
The room behind the door has a ladder leading down a shaft with three circular saw blades, which are as yet inactive. When you reach the third, backflip onto a ledge behind you and shoot another large vase (containing—surprise!—a big medipack). Drop down to the floor.
Run past the two swinging chains into a large room with three empty pits flanked by two inactive cog wheels; shoot two bats. The door in the S wall is closed, but there’s an accessible passage in the N wall with more chains to dodge. After a left turn at the first you reach a T-intersection; the right-hand corridor ends at a closed door, so turn left, and proceed with caution past the next chain. Shoot another bat.
On your right is an alcove with a small medipack on the tile behind a spike pit. As soon as you enter the sloped corridor ahead, you trigger a rollingball. Dodge it by swerving into another alcove on your left (or hopping back around the corner), then go up the slope and use the lever around the corner. It opens the door on the other side of the T-intersection. Go through that door, shoot two more bats and open the floor trapdoor (it has a handle to indicate the place where Lara needs to stand in order to do so).
Drop down the shaft and into the water below. In the N wall there’s a closed door underwater, and an open doorway immediately above it. Pull up into it and use the pulley. You get a camera view of the big double doors back in the hub room opening. The door underwater is open now as well, so drop back down into water and let the current carry you into the next room.
Pull up out of the water, jump up to the ledge jutting out of the wall and use the lever there. It causes a block to move out of the opposite wall, as shown in a camera view. You need to hop back once to get off the camera trigger. Jump up to grab the monkeyswing and move over to the block that came out of the wall, then take a running jump with grab to the next one and pull up into the crawlspace. Light a flare and move in the direction of the squeaky noise.
That noise is produced by a cog wheel up in a dark alcove on your right. Climb into it and pick up shotgun ammo for a secret (2/3).
Follow through to a rocky cave, where you are greeted by a different kind of squeak (another bat for you to shoot). Pull up through the hole in the ceiling and use the jumpswitch on the wall straight ahead. Go through the open door and find yourself back in the large room with the three pits (now filled with water) and the two cog wheels (now turning). Of course the circular saw blades in the shaft that you need to go back up are now active as well: climb up and wait immediately below the blades for them to pass; you have sufficient time to move out of their range before they come round again. When you’ve made your way to the top, retrace your steps to the hub room (just turn left where you placed the Owl Coin and follow the passage) and go through the large double doors.
In the room behind it is a large vase (containing nothing) and another non-interactive scroll on a pedestal, but at least you can get rid of the Gemstone you’ve been lugging about. (If you happen to have missed it, you need to backtrack. Refer to the ‘Gemstone’ section above.) In the next room, all you have to do is run up the passages in the N and S walls and use the pulleys located up on a balcony on either side. This opens the next set of large double doors in the E wall (there’s another one in the W wall, but that one’s just for show).
In the corridor that follows, turn left if you want more pickups, or right if you want to skip them. If you go left, drop down into the pit (the rooms may black out here from certain angles) and get the shotgun ammo lying on the ground. Stand in front of the crawlspace and wait for a scorpion to come out before entering it. Inside is a small medipack and another scorpion. Climb back out of the pit.
For the last secret (3/3), climb the ladder immediately around the corner, on your left. In the room on top, shoot the large vase (containing a small medipack). Go back down.
Follow the corridor to the other side of the pit and enter the next room. It contains no fewer than four large vases (containing nothing) and another scroll on a pedestal. This one, however, can be picked up, because it is Demetrius’ Scroll, which is apparently the thing you’ve been after all along. Picking it up opens the door to the next and final area.
Slide down the slope into a cave with a small underground lake. Follow the tunnels until you enter a room in which you are accosted by a hammerhead shark. There is a safe ledge for you to get out of the water in the NW corner of this room (well, reasonably safe, as the hammerhead turns out to be pretty determined in getting a bite out of you). There is also an extra big medipack on another ledge at the E wall. Back in the NW corner of the cave, pull up into the shadows, slide down a slope and follow the passage to end the level.