You start out in the chapel room. There are some items here you can grab, most notable ammo and a Green Gel jar by the pulpit. Talk to your partner on the first pew to start a cinematic. When the cinematic is over, you'll find yourself in a totally different area of the church. Take note of all the white urns on the ground. These are the church's equivalent of boxes and barrels, and can contain the same things. Move around the room with the walls and walls of candles, smashing urns as you see them, and you'll come across a jar of Gel on the far side of the room, and some pistol ammo on a stone mantle. Watch out for the bomb near the Gel. There's also some parts under a shelf in the center of the room, near the large stone table with bones. To leave, walk up to the large painting of a pastor and head left. There are more urns here to smash, assuming you didn't smash them earlier. Walk up the stairs to the next area.
The next area is a long hallway. To the left is a brightly lit room with two Haunted, one of which will most likely be a fat zombie or zombie priest. They will notice you if you try to sneak into the room, so have them follow you (there is a zombie at the far end of the hall, but he doesn't notice if you engage these enemies) and shoot them with a flash bolt, blinding them so you can sneak kill them both. Enter the room they were guarding and smash more of those urns and grab the other item and sneak down the hall to the other zombie. Wait till his back is turned, then sneak kill him. Take a right and look around in the room for some supplies, then head forward to a carved door. Examine it to make the detective pry out the head of the carving, which will place it in your inventory (it will be used later), and the wall will rise, granting access to a new area. Grab the note and pick open the safe on the table for a key.
Walk down the stairs (no need to sneak, there are no enemies here) and smash the urns on one side of the stairs. Break the chain holding the iron doors closed, then enter a large room. Notice the large carving in the center of the room. It is missing three heads, one of which you happen to be holding. Place it in the center hole and the two large doors on either side of the room will open. Before you go any place else, notice the save room door. Enter and enter the mirror, and do the usual (history items, keys, upgrade chair, saving the game) then exit. Enter the left side, first. Doing so will unlock a room full of Gel in the right section. It will be missed if you go right first, as poison gas fills the chamber where the goods happen to be stored.
First off, you will notice gray fumes shooting from a blue pipe. These are toxic gasses that can kill you quickly. Seal off the valve (with the crank) and grab the Syringe nearby and enter the door.
There are three two-headed zombies roaming this hall: one hanging from a rope, one locked in a cell, and the other that will come from a set of doors when you enter the hall far enough. Stay at the beginning of the hall and whip out the Sniper Rifle, shooting him in the head. Note that if you hit him on his "secondary" head, even exploding it, he will still attack. Shoot the normally placed head for kills. Sneak kills are not available for this type of enemy, as well, so don't waste flash bolts. Note that when he drops some spikes come from the walls, so stay put. Walk slowly forward until the doors open and the other zombie comes out. Shoot him in the head, too, and then turn back to the locked cell. The enemy within will most likely notice you, but there is nothing he can do about it, other than run around in the small room. You can shoot him in the head if you want the Handgun ammo on the bench in his cell. If not, leave him in there (not breaking the wood beam keeping his door shut), and head into the cells on your right, picking up ammo and such. Don't enter the area where the second zombie came from, yet. Instead, sneak up the hallway and disarm the bomb for parts (you will need that later on), then notice the lever on the right, and the ceiling of spikes down below. There are two or so of those two headed zombies down there, so activate the lever to pulverize them, saving ammo and trouble. Head back the way you came and enter the door you avoided earlier. Go all the way down and enter a door. Now you're in the room where you unleashed the spikes.
Don't worry about the spikes (they won't come down), and harvest all the Gel they left behind! There may also be some ammo to be had.
Back in the room with the lever, notice the red glowing room. Sneak in, because there is a bomb attached to the back of the bloody chair. Disarm it, then grab the Grenade. Take a left and notice the ladder set into the floor. There is also a safe there, but it is wrapped with barbed wire, so you can't pick it or anything. Enter the manhole and head down the ladder to show you a scene of The Keeper locking you in with a room full of toxic gasses. Grab the item on the table to the right, then notice the crank valves on the pipes. Crank away on them, but be wary of the lone two headed zombie which will try to interfere with your plan. Do away with him (he will most likely attack when you start cranking), then crank on until the gas clears the room. Now head down the ladder. There's some pistol bullets on the railing by the ladder.
The hall has a bomb at the end of it, as well as two of those two headed zombies. One stands there past the corner, and the other patrols. You can get them both by shooting the bomb for some initial damage, but you will need those parts later, so instead get their attention one by one and kill them with headshots. If they get too close, you can run for the ladder and climb to the other floor, as they can't use it. When they are gone, grab the Gel jars on the side of the hall and disarm the bomb, then head forward.
In the next room is a tripwire between two candle-like pillars. You don't get parts for disarming these, but do it anyway (setting off tripwires is always a bad idea). The hallway beyond has three more zombies, but they crawl on the ground. You can stomp them if you're quick, but they like to latch on and bite your ankles, which does a surprising amount of damage, so you may just want to shoot them from a distance. You can also shoot freezing or shock bolts, then stomp them to death. Head on and use the crank to open the door (and closing the door to the area you just came from), then enter. Keep going until you find another of those carved doors with another face you can carve out. The wall will rise, leading you back to the room with the save room again. Place the new face into the block, then head right.
Unlike the left side, you can use the spike ceilings on the right side to kill most of your enemies, but you'll need some timing and dexterity to get it done. Enter the first room, and get the ammo and parts by the downed spike trap, then listen to the audio tape. There is a statue on one of the pillars by the hanging spike trap, so shoot it to gain a key. Enter the next area and disarm the tripwire, then look across the chasm. There are two cranks that you cannot reach, but you can shoot them. Shoot them from the doorway and watch the spiked ceiling fall down. Get on (it acts like an elevator) and ride it as it slowly rises. A two headed zombie will fall from above, and you have to kill him. Kill him however you wish, as it shouldn't be difficult, as he only crawls along. When the trap stops moving, hop off and enter the next area.
In the next room, there is another of those hanging zombies. If you get close enough, he will drop down, then a few seconds later the spiked ceiling will also drop down, so don't go into the pit! Don't ride it up, either, as above the trap is a fixed ceiling of spikes. If you want to kill him without expending ammo, run around until the trap rises to the ceiling again, then run over through the pit and yank the lever right away, as the trap takes some seconds to deploy. You could also just shoot him from a distance with the sniper rifle and not worry about timing the trap. Smash the urns and enter the little tunnel to grab some supplies (beware the tripwire on the other side as you exit), and now you're on the same side as he is/was. Note the double door to your left. You can't open it right now; its locked. It will unlock when you start cranking on the wheel to open the gate, however, and five of those two headed zombies will come pilling out. You have two choices here: You can either shoot them all with the Shotgun or Explosive bolts from the Agony Crossbow (but try to save those for later), or you can try to lure them a few at a time to the pit and deploy the trap on them. Whichever you choose, after they are dead meat, enter the room they came from and grab the shotgun shells and head down the dark stairs.
As you walk along, keep the Lantern out, because while there are no enemies to worry about, there are plenty of tripwires and bombs. Disarm them all, and follow the path to the end, where there is sort of a shrine with dead bodies aligned in a circle with lots of Gel and a key statue! Grab them all and head back.
Open the gate, then head on until you find another pit with hanging spikes. There is one of the too-common-in-this-chapter zombies on the floor, and he will get up if you shoot him or get down into the pit with him (you need to do this, as the carving is on the other side). There is also another zombie that loiters by the carving, near the lever. You can try to lure the other one into the pit while avoiding the attacks of the other, then jumping up and pulling the lever (it takes a lot less time for this one to drop than the previous one), or you can just shoot the one in the pit, then hit the other with Grenades. Its up to you. However you do it, before you interact with the last carving, look up past the hanging spikes. You'll see a little opening. Pull the lever (have the lantern on) and hop on when the spikes start to rise again. Be sure to jump into the little opening or you'll get speared by the spikes on the ceiling! In the little room are Flash bolts, Gel, grenades, and a key in a little golden box in front of a (non-breakable) statue. To leave, head around the corner (not to the spikes) and kick down the ladder. You'll end up where you first saw the pit. Walk to the carving and get the last face in the wall, and it will rise.
Enter the last carving and then watch the whole thing rise. Use the save room (recommended), then hop down into the hole. The giant carving slams down behind you, so there is only one way: forward. Wait as the giant door opens, then you'll see your last nemesis for this level: The Keeper. He'll whack his head with his hammer, taunting you, and the game shows you that the ceiling is COVERED with spike traps. Unfortunately, the detective steps in a trap as he backs up, and now you've got to get free.
Follow the on-screen instructions to get out, then run back. When you're ready, run forward. The game switches from the usual third person camera to forward facing, so you can see the glowing red spots on the ground. These are more of those traps, so weave around them as you sprint. There is no need to worry about stamina, so hoof it! Get caught by a trap and you'll have to do the sequence all over again.
Once in the next room, see Leslie in a cage. After the cut scene, grab the supplies and exit, then watch another scene of Leslie fleeing down some stairs, leaving you behind with boxhead. The game makes a checkpoint here, as the detective pounds on the door. You have to kill boxhead in order to open the door to the next room. If you have any explosive bolts, use two of them to kill him. If you pin them right on him (don't shoot for his "head," it is metal and you don't do any bonus damage), only two will take care of him, but if you shoot them only close to him, it will take more than two. If he happened to drop his traps, make sure he's gone, because they take a lot longer to disarm than tripwires or bear traps. Now that the other door is open, a dial appears on the right corner of the screen. This means that, while the needle moves slowly, the room will eventually fill up with toxic gas, and you'll die. Also note the amount of boxneads in the room you are currently: 15. There are even more scattered around in the adjoining rooms, so don't even think of trying to out-gun him and kill him that many times. He appears every time you crank on a valve (to try to stop you), so you can walk around in the rooms that are open to you without having to worry about him, save the last part.
Enter the room adjacent and get the ammo and syringe off the table. Also note a spare crank valve. Keep a mental note on its location, then crank on the nearest valve. Boxhead will start to spawn as soon as your hands touch the thing, so mash the activate button as fast as possible until it is sealed, then sprint into the round doorway. You can also shoot an Electric or Freezing bolt into the corner before you start cranking to keep him in the corner for longer (it will active when he is fully spawned), though his spawn time is rather slow. Once you are beyond the door, there will be a scene of the detective sealing the door shut, keeping boxhead out. As the room doesn't have any enemies in it (yet), grab the ammo and parts, then crank on this valve, too. Once you're done, you'll get a cinematic of boxhead ripping off his own head, so he can spawn into the room with you. Have harpoons or two more explosive bolts ready, as you have to kill him in order to escape the room. Once he's dead, exit and head left.
You'll get another scene of boxhead wrecking the last valve (he really wants you to stay). Don't waste ammo on him. Run to where you saw the spare crank and he will follow you. Have a freezing or shock bolt ready, then shoot him with it when he's in the room with you. Once he's immobile, run back to the valve. Before you replace it, shoot another freezing or shock bolt into the doorway, so you can crank without being attacked. Crank as fast as you can, then run down the newly opened hallway and through another round door, where the detective will seal off boxhead again. Climb up the ladder and get attacked by a lone boxhead. It will wrap its tentacles around you, so follow the on-screen instructions to shake it off and toss it into the pit. There will be a cinematic of the detective yanking the lever to finally kill boxhead. After its over, Sebastian will state with relief: "Finally!" and you likely will, too.
Follow the new area to the end and turn right. There are the darkened stairs that Leslie ran down earlier, with one of those large Gel jars near the top. Walk all the way to the left and before you head to the cave entrance, there's another Gel jar hidden behind some rocks. Enter the hole in the wall and down the ladder and it is the end of this chapter!