You are looking for 4 keys that take you through the level, and a gold key to exit (5 keys altogether).
You can do a safety drop to the room below, with your weapons drawn to kill a dirty, great rat! Ugh! The movable block needs to be pushed twice into the next room, and then once to the right to stand under the lever. Pull the lever to open the room door and a trapdoor below. Kill the rats that enter through the door before you jump down and retrieve the large medipack from behind it.
Drop down through the trapdoor beside the block to find yourself in a huge room with several doorways and ledges and a pool of waterat the bottom…This is the Cistern. Have a good look around. (Spooky!) You’re going to be in and out of this area, so try to get your bearings before you start.
Firstly, look to your right to see if you can shoot a couple of rats walking around on the ground floor. There are a couple of crocs in the pool, which you may be able to get from the edge of the ledge you are on.
Also to the right, you'll see a small raised platform…run and jump over to it and kill another rat. Walk down and to the right. There’s a crevice on the wall which you can grab by pressing up and action. Shimmy left until you come to a block with some shotgun shells…Take them and then jump to grab the next crevice and shimmy left until you can drop onto the elevated walkway below.
Turn around and walk to the end, drop down onto the next walkway, follow it and climb up to the save crystal and save your game. From the top of the platform you are standing on, you can jump to the balcony ahead and climb the stairs with pistols drawn! The charming Pierre is waiting for you, but if you can manage to get behind him while you fire, he will run out of the door and disappear…until the next time!
There’s a lever in here that floods the cistern, but you don’t pull this yet as you have other things to do! Turn right at the entrance and climb the block to the far right of the room. Turn around, jump and grab to the next ledge. Pull yourself up and run, jump and grab to the next high ledge, which has some goodies; a large medipack and some shotgun shells.
Now you need to go back the same way, to the other side of the room, taking a standing jump to the lower ledge to avoid losing life. From that ledge, turn right and step up to the next one, and in the alcove above there is small medipack. Climb up.
There’s an opening above and to the right of the alcove that you may have missed if you don’t know where to look…this is the level’s first secret. Climb all the way up to find some shotgun shells. There’s a slope ahead…when you slide down it, you need to jump just before the end to grab the platform above the slope, where you’ll find more shotgun shells. Pick them up then slide down the slope to get back down.
Make your way to the opening in the floor with the water…and see two rats! See if you can kill them from where you are. You may have to draw them out by jumping over the grating. Once they’re dead, jump into the water and swim through the tunnel. Surface as soon as you can, climb out, then draw your pistols for another rat.
Climb the stairs and run, jump and grab the grey block, then again to the ledge ahead to find the first rusty key.
Keep going to a pit with another two rats…these should be easily dispatched from above. Turn around to stand with your back to the edge, drop, and grab the edge by pressing action, then shimmy left, dropping down to the green floor at the end and slide down the ramp to the main room…you can find another save crystal here.
Drop down, turn left, jump and grab the bridge ahead, turn left and pull up to the next block. Pull up onto the long walkway above the shimmy ledge, making your way back to near where you first started the level. Here you'll find the second rusty key. Pick it up, safety drop down onto the ledge below, make your way to the end, then run and jump back to the ledge you first started from. Walk to the end of the ledge to the right, before taking a standing jump over to the next, or run and jump if you feel safer! Face the first door to the left, then run, jump and grab the ledge in front of it. Set yourself in front of the keyhole and press action to use one of the keys to open the door.
There are two gorillas inside, but they come from somewhere near the back of the room, so you can either pick them off from where you are, or turn left and climb up the ledge behind the door and get them from there. From that ledge, take a running jump over to the next ledge, and draw your pistols to see Pierre off again! (Ooh, he's such a nuisance!) If you jump up and down as you shoot, he may not hit you so much, and he'll eventually disappear again.
When he's gone, run, jump and grab the third ledge, where there's a large medipack, then again to the next ledge. OK, there's a ledge in front of you now, in an opening in the wall ahead. It's not a secret, though there are some goodies there, but the ledge is a bit of a pain to get to! Walk to the edge of the ledge you are on, turn around, walk one step forward, then back-flip onto the ledge (video). Pick up the Magnum clips (or the Magnums if you don't already have them). Are you okay? Good...now we can carry on!
Once you've picked everything up, you can run, jump and grab back to the ledge you just flipped over from. Turn around, then run, jump and grab to the wall ahead (above the alcove)...Lara will grab a crevice and you can shimmy along to the right. Drop down and travel as far as you can, stopping after you've slid down the last slope to kill a croc from where you land. Jump down, then climb immediately onto the block where the save crystal is to get another croc! (You may want to use your pistols if you are out of reach of the beasties, to save ammo.) You can either save your game here, or wait until you've done the next little bit.
There's a passageway to the right of the slope you came down...with another croc hiding there! You can draw it out by walking down a little way, then back-flipping as you shoot! (You can usually hear the crocs before you see them, which helps a bit!)
There's a large medipack,and another rat, down the passage where the croc just came from. Retrieve it, and climb back onto the block with the save crystal...save your game if you haven't already, climb up the next block, turn around and jump and grab the small ledge ahead. Take a running jump to grab the ledge in the corner, turn right and run and jump to the next ledge. Jump down to the ledge below, turn and jump up to the crevice in the wall, shimmying left until you reach the doorway. Pull up and pull the lever to open the door. There's a spike-trap and two rats below. Shoot the first one, jump over to the opposite ledge, jump over the spikes to the floor and kill the second one.
Carry on until you come to a green, mossy room with a block you can drop down onto. There's another rat you may be able to get from there. Go through into the next room with weapons drawn for another rat. Do NOT drop into the water of you will have to go all the way around again! Climb up the stairs and then up to the next two ledges. Walk to the end, turn right then do a running jump to the ledge below. Climb up the next ledge, then jump and grab to the middle ledge. Jump over to the alcove and pull the lever to open the door below.
The first silver key is behind it, and when you pick it up, the lovable Pierre appears yet again, so as soon as you stand up, roll, draw your pistols and fire until he clears off! (Boy, are you just itching for the showdown!!!) When he's gone, swim through to the next room and climb out. Look around...there are a couple of rats on the ledges above, and a high lever with a door near it. You can't get to the lever yet, but you may be able to get a rat by climbing on one of the blocks and jumping up and down with your pistols.
Jump into the water and follow the next windy passage to the main room...this is where the crocs were swimming at the beginning of the level...you did get them then, didn't you? If not, surface and climb out as fast as you can and kill them from the side. Swim right through to the large area, below where you first entered the level. Under the water is a U-shaped passage with an opening at either end. Swim through one of the openings to find some Magnum clips which are the level's second secret.
Climb out of the water and find the ramp set between two gargoyles (screenshot). Climb up to the walkway and back to the balcony (screenshot) that leads to the room with the lever to flood the cistern. This is where you first met Pierre in this level. Straight opposite the doorway is a lever set into the wall. Pull the lever then return to the now flooded main room and swim back down and through the passage you came from, back to the room with the high lever. Climb out and kill the rat(s), pick up all the goodies on the ledges (a small medipack, a large medipack and shotgun shells and use the save crystal. Now you can jump back into the water, pull the underwater lever and retrieve the second silver key from behind the door that opens below. Exit back to the main area through the door the opens when you pick up the key, and this time return to the second door (across from the bridge near the beginning of the level) and use the second rusty key to open it.
Jump into the water past the now harmless spikes¹, turn left, swim down...go through the opening on the left, pick up the small medipack and the gold key, then exit through the door that opens.
There's another croc knocking about now, so climb out ASAP, kill it then head over to the ornate balcony with steps leading up to it. Climb up but before you open the door, look to the left. You'll see a block of stone that looks somewhat different to the rest (screenshot)...push it, enter the room and walk over to the balcony at the far end of the ramp. Stand with your back to the ramp, as close as you can, flip back and keep hold of the jump button until you reach the balcony. Here you'll find a large medipack and two sets of Magnum clips - the third secret.
Return to the locked door outside and use the two silver keys to unlock it. Climb up onto the block to the right and use the save crystal to save your game. Jump and grab to the ledge above the door through which you entered. Opposite you'll see a ledge with a gold keyhole, but there's a nasty gorilla waiting there! To kill it in relative safety, run and jump over, safety drop to the floor, climb back to the ledge above the door and pick it off from there, using your pistols to save ammo.
Once you've used the gold key, the door below opens and two lions emerge...shoot them with your pistols from where you are, or from the ledge above the door, then safety drop down to the floor. Through the door you'll see a large room with a chequered-tiled floor. Some of the tiles are broken and have spike-pits beneath...but there's one at the left corner that conceals a small medipack! Get it if you want all the pick-ups.
I normally just run and jump over the floor to the back of the room as the tiles take a second or two to actually break completely! You'll find a lever there that opens a door to the right...inside are three lions, but there's no need to go in there unless you want all the kills. If you do, pull the switch, climb onto the block and pick them off from there.
Go around to the right of the switch and pull out the movable block, twice. Go around to the side, pull out the block toward the big room as far as you can, then go around and push it once. Climb up it and up to the balcony to kill two rats and collect more goodies, two sets of Magnum clips and another large medipack. Climb back down into the room with the chequered floor and go behind the block you moved to find a pit filled with water...jump into it to exit the level.
¹It's possible to get down here without flooding the cistern, but if you do, you won't be able to get out and will have to load in your game from your last save point or restart the level!
If you missed anything while playing the PC version, here is the savegame file with all kills, pickups and secrets.