When you begin the level, you are on a walkway. To get to the first secret, turn to the right, run down the walkway, and jump to the pillar. Jump on the slope and back flip to the catwalk. Jump over the wall to retrieve your save crystal and item. Use the block to climb to the roof. Run and jump across to the crane and run down the walkway. Drop to the counterweight, jump to the ladder, and make your way down to the ledge. Walk through the barbed wire to the doorway. Dispose of the big rats and find your next secret.
Return back to the start of the level and drop down to the ledge below. Drop to the slope, back flip to the ledge, and watch for the crow. Pull the switch and jump across to the ledge with the small medi pack. Then, drop down to the door and jump to the ledge. Be careful, because one half of the ledge is a collapsing ledge. Run and jump across to the balcony.
Make your way down the hallway and dispose of the big rats and the sniper. Retrieve the flue room key and return to the balcony. Drop down to the platform below and push the switch. This will drop the gate to let you leave the room. Run and jump across to where the trapdoor dropped, jump up, and leave the room.
Once you're back outside, jump up against the wall and grab an invisible ledge. Shimmy to the left and enter the crawlspace. Crawl to the other side and push the button. Then, backtrack through the crawlspace to the balcony. Drop down and slide down the roof to the boxes below. Have your guns ready to take out the snipers and retrieve your save crystal and flares.
Scale your way up the boxes and ladders back to the beginning. Jump across to the ledge to where you picked up the medi pack earlier. This time around, shimmy to the right on the invisible ledge to the end of the building. Climb up and follow the path, taking the lower path first. At the end of the path, you can see a scaffold. Jump to the scaffold and enter the flue room. Push the switch and carefully make your way back to the scaffold. There are flames on the way back, so time your moves carefully. Hang and drop from the scaffold to the ledge below. Enter the crawlspace and go through to the other side to the balcony. Shimmy to the right and find the secret. Make your way back to the end of the building; this time taking the higher path. Jump to the ledge and grab the ammo. Then, drop to the ledge and retrieve the rocket. Climb back, drop down the hole, and follow the hallway. Drop down the hole and make history of the sniper. You are now in the water control room.
There are three switches, two of which are in glass cases. Pull the one that is not in a case and make your way down the hallway to the second room, where the switch has made the water rise. Jump into the water and swim down to the switch. Pull it and backtrack to the water control room and pull the same switch again. Run down the hallway to the first room, where the water is now only half way up. Jump into the water and swim down through the trapdoor on the floor that opened when you pulled the switch in the water. Swim through the under-water passage and through the propellers that are now turning slowly. Make your way to the surface at the end of the room and climb out of the water. Eliminate the enemies and light a flare. Go down the hallway and into the crawlspace on the right to the sewer.
Now, you must stay out of the way of the sewer cleaner and move the mesh box over to the electrical panel. The cleaning machine will make its rounds, run into the mesh box, then run into the electrical panel. This will remove the glass from one of the switches. Climb up the ladder into the water control room and push the switch that is now accessible. By doing this, you will fill the room through the red door with water. Pull the other switch to fill the tank with water, make your way down the hallway again to the last tank, and swim to the red room. Swim to the ledge on the other side of the tank and watch out for the guard. Run in and push the switch. Return back through the door, but don't try to jump into the water - it's now gone. Monkey climb across the room. Pick up your save crystal and return to the water control room, where all three switches are now accessible.
Push the button that has just been released from the glass. Return to the red room and swim down to the trapdoor that has opened; then, swim to the surface and expect some action. Climb the ladder and walk through the barbed wire. Jump to the platform on the crane, then run and jump to the ledge across from you. Walk through the barbed wire and shoot the snipers. Enter the hallway and move the box into the center of the room. Climb up to the ledge and jump across to the next ledge. Here, you will find the cathedral key. Go to the open spot in the railing and down to the door where you will use the key.