The Hall of Seasons (All Crystals Completed)
You've placed all four crystals in their appropriate slots. Return to the room's center and press the floor switch with the water symbol on it (the wavelike lines). Go to the opened doors and pull the middle lever. You will drop down into the torture zone. This is where you would have fallen if you hadn't pulled the appropriate levers when hunting for the crystals.
Follow the corridor to the intersection. When you reach the intersection, you should have a clear path on the left and a series of obstacles on the right. You can traverse the obstacles on the right (stay away from the vertical blades, and do a running jump past the block) and find a health pack in an alcove to the right. Otherwise, go left at the intersection.
Watch out for the blades that come down from the ceiling. The next area has a blade that rotates. Wait for it to rotate horizontally and then scamper underneath. Leap over the pits and grab some ammo from the alcove on the left. Continue left through the torture area. The next catwalk has spikes that appear from the left and right walls. Adjust your camera view so it's somewhat overhead. You can then more easily maneuver Lara in a left-right pattern around the spikes.
Leap over the pit and go through the door on the right. Descend the staircase. Pull the lever to open the nearby door. This reveals a brand-new chamber.
In this area, you must turn four valves, all labeled with the symbols from the Hall of Seasons. Turn left immediately. There's a valve close by. Turn it. One down, three to go.
Return to the start position. Perform a running jump to the next platform (it's tough, so use the action button to grab hold of the edge as Lara jumps). Go to the ladder on the end of the platform. Climb up. Turn the lever at the top. Two down, two to go.