Climb up N and use the rims on the wall to get to the alcove in the S wall. The lever there opens the door W.

Here you can use the rim E to get to the other side (down arrow and release Ctrl when Lara has her feet on the rim, then Ctrl again to grab the other side). Shoot the rats climb down and down and up again. In the far end of that corridor is a Small Medipack. Go up the steps W.

In the next room is a receptacle block in the middle. In the NW corner is a crawlspace where you can get Secret #1, the Shotgun. Get back out to the receptacle room.

The Red Key.

Go into the passage N, get down the stairs and into the water in the NW corner, in the W wall is a small tunnel, swim through for Secret #2, there is a crack up W, grab it and go left all the way. Jump back (down cursor) and use the lever. The door downstairs opens up, go in for your prize, a Medipack. Go swim back and left, then keep going right around corners till you swim into an opening under the Obelisk island. Use the under water lever there to open a trapdoor.

Get back and swim S and find that open trapdoor, down there is another lever N that opens the gate on the left. Swim through and left and climb out. Get the Shotgun ammo on the ramp and side flip left to avoid the boulders. Once up you are on a ledge in the previous room, and use the rims on the N wall (grab up to the first alcove above, shimmy right and drop again at the last alcove) to get around to the S. Jump from the pillar to the plateau.

Pull the lever, watch the flyby and jump in the water below. The door N is now open. Swim to the SW corner and climb the platforms, grab up to that rim S. Go left and up and down and when you are at the E wall, up again. Then go to the right as the opening where you have to go is in the corner there (SE).

Go in and to the left, left again, over the roof into an opening and use the lever. This will trigger an earthquake and the roof has changed, the sand dropped into the pool below. From the roof you can jump down into a deep part of the pool. Now you can climb up N and get the Red Key.

Swim back to the S and keep going to the receptacle room in the S.

The Blue Key.

Head into the opening N and into the water, swim up and get out at the other end to shoot the croc. Then dive back in and use the underwater lever N. The door N opens. Go in and open the coffin for Shotgun ammo. Jump up SW and find the lever on the N wall. Avoid the blade and then go SE into a crawlspace for Secret #3, some more Shotgun ammo.

Go back out of this place to the pool room and into another door on the other side S. Lower down into the water and then go through the opening W.

Get the Blue Key from the pedestal and swim back, climb out NW, jump to the block in the water and grab up to go back to the room with the pool. Swim back through the pool and when you are out go to the receptacle room at the other end N. Go in and place the two Keys on the block there in the middle.

The Torch.

The door W opens up, jump down in the opening and swim to the end. Use the timed underwater lever and swim up through the trapdoor. A panther is waiting for you there. Go to a climb block S and jump to the pillar and shimmy around and jump into the opening N. Avoid the axe and use the lever, this raises a block in the floor near the pillar.

Go NW and into opening and push a block over the raised block (the long way around) onto a marked tile so another block will rise on the E wall. Take the Torch that is near the W wall (left of the door) and ignite it in the passage N, jump on the block W and ignite the wall torch. The door W opens, you can leave the Torch here.

The Artefact.

Go up the stairs and get Shotgun ammo and a Small Medipack. Go in SE for Secret #4, a Mummified Cat. Go out again and continue N, shoot a Demigod in the next room.

On the W wall is an Egypt Artefact, pick it up and a rope appears. Look NE and go to the pillar that has a crack, get there via the block with the wall torch. Shimmy around and jump backwards to the next pillar, from there to the rope. Turn around 180 degrees, turn a bit left and use the rope to get to the pillar with the crack so you can end up onto the ledge at the E wall (I could swing through the gap between the pillars and end up on the ledge in one go). The lever there opens a trapdoor on the ground floor in the grated floor.