Part 11

Turn right at the shaft intersection to find armor and a bullet clip near a dead security guard.

Go to the other end of the shaft, break a vent grating, and enter a room with barnacles. Defeat them and collect nearby supplies.

Find a ladder near a barnacle. Climb it and drop onto the shaft to the left, or jump to kill the barnacle and proceed.

At the top of the curving shafts, crouch-walk to a large shaft opening and jump in.

Move slowly along the dark shafts with your pistol and flashlight on, shooting headcrabs.

You will reach a point where section c1a2b loads again.

When you reach a grating, bash it off. Wait for the guard below to shoot a zombie before dropping down. (Note: The zombie may kill the guard if not distracted by moving to the edge of the shaft.)

Approach the guard to trigger dialogue, then talk to him to make him follow you. You can return to previous areas, including a secret ammo room, through the door the scientist opened.

Head up the stairs and have your guard kill the headcrab at the top.

Walk down the wide corridor until a zombie drops from above. Let your guard kill it.

Move forward until another zombie breaks through a wall. Let your guard kill it.

Go through the zombie's wall hole to find hand grenades.

Crouch-jump through the office window a scientist broke out of to find shotgun ammo and health packs.

Crouch-jump back through the window. If the security guard is still alive, whack him for his pistol clip.

Ascend the shaft past the jammed elevator doors. Quicksave.

Take a running jump through the elevator doors across the shaft to the ladder on the wall. If you miss, quickrestore and try again.

Climb the first ladder and get off on the ledge to the right.

Climb the next ladder and get off to the left.

Climb the third ladder and get off to the right.

You are now on the highest ledge. Quicksave.

Jump to the ladder on the side of the elevator car.

Climb onto the top of the elevator car.

Find a bashable grate on the elevator car and drop through it.

Move against the elevator doors to make them open and take you to the "We've Got Hostiles" chapter.

As soon as you exit the elevator, run right and wait for a scientist to pound on control room windows. Hit him once with the crowbar before he runs off, ensuring he goes towards the control room door (position yourself to his left before hitting him).

Enter the control room, use the big red button to open the silo door, and proceed to the "Blast Pit" chapter. (This trick works better in the Source version.)

Watch a scientist run into a laser tripmine. Use nearby first aid and HEV stations if needed.

Go past the fire door, optionally bashing its button's glass cover.

When you reach two red laser alarms, run through them, then crouch and move against the right wall to avoid the floor turret.

Crouch-walk to the corner; a headcrab will teleport in. Back up while the turret shoots it.

After a second headcrab appears and is shot, crouch-walk to the wooden crate ahead.

Strafe slowly right until you can see one leg of the turret. Shoot it with your pistol until it falls over. (The second headcrab may not teleport until you move further into the room.)

Get the MP5 assault rifle near where the headcrabs teleported. Reload it immediately as it starts with half a clip.

Bash non-explosive crates to find armor and shells.

Go to the other side of the fencing to trigger a headcrab teleport. Run back around the fence to shoot it.

Wait for two more headcrabs to teleport in for easy shooting. (You may need to move around in the area past the fence to trigger them; up to four more may appear.)

Go through the fire door without bashing its glass cover.

Jump over the first tripmine and crouch-walk under the second.

In the next corridor, move forward until a vortigaunt teleports in. Quickly go back to the fence past the fire door.

Wait there; two vortigaunts should kill themselves on the tripmines. If not, get their attention again or shoot them.

After two vortigaunts are killed by tripmines, a third will be in the hallway. Shoot it with your assault rifle. (There may only be two vortigaunts in total.)