Part 16

Return to the firing chamber. Go along the walkway to a room with a red switch; use it to change which door is open, returning you to section c1a4b.

In section c1a4b, turn right after exiting the door-switch room. Shoot the explosive crate near the bullsquid, then finish it off if needed.

Shoot the explosives on the nearby path until they detonate. Crouch-walk across the surviving blue pipe.

On the other side, break open two non-explosive crates for health packs. Move down the corridor until section c1a4d loads.

In section c1a4d, ignore the ladder. Jump over the railing next to it. Run for the large floor grating ahead, bash it open, and descend the ladder.

At the bottom, get onto the large brown pipe and move along it until it divides.

Go down the left pipe to a short ladder leading to a bashable grating. Shoot the zombie below through the grating with your pistol before bashing through and climbing up.

In the room beyond, find and resupply from grenades in a small nearby room. Move down the corridor until section c1a4e loads.

In section c1a4e, use the opening control on the wall to the right of the closed door. Toss a grenade at the two zombies just past the door, then finish them with the shotgun if necessary, or try to run past them.

Move around the giant fan shaft's upper walkway to find a ladder to the lower walkway. Climb down to a narrow platform leading to the fan's button panel. Quicksave, then activate the fan and immediately run backward up the ladder to avoid being hit.

Jump into the space over the fan to be blown to the top of the shaft. Bash through the boards.

Move to a wall with a large vent covering and bash it out. Toss a grenade into the vent to deal with the headcrab inside.

Drop into the vent shaft and move along it. Shoot a headcrab through a large metal vent covering before bashing it away.

Past that, shoot a headcrab through a board barricade, then bash through it to return to section c1a4d.

In section c1a4d (Fuel Room), bash through metal vent coverings and drop down next to the fuel room.

Engage the enemies inside: headshot the first two zombies with the pistol, then use the shotgun on the third. Shoot the headcrab from a distance with the pistol.

Use the control panel left of the headcrab's machine to press "OXY ON" and "FUEL ON".

Find the ladder in the alcove to the right of the headcrab's machine and climb it. Use the control on the wall to open the door, then head down the corridor on the right to return to section c1a4b.

In section c1a4b (again), return to the blown-up walkway and cross it by crouch-walking along the blue pipe.

In the door-switch room, hit the switch to be returned to section c1a4i.

In section c1a4i, go past the doorway to the rocket firing chamber to a red railing. Jump onto the railing and crouch-drop to the floor below (losing about 20 health). Alternatively, try to hug the wall from the left end of the railing for less health loss.

As you go along the walkway, you'll come to a wide gap. Quicksave, then take a running jump across the left side of the gap.

Get out your shotgun and run forward until you reach a zombie. Quickly kill it with a double-barrel shotgun blast to the head.

Enter the door-switch room past the zombie and hit the switch to return to section c1a4b.

Exit the door-switch room to find yourself in the area with the bullsquid you killed earlier.