Part 18

Explore the Ratman Den, a hidden area hinting at other trapped individuals.

Look for a red X on the glass, shoot a portal there, then shoot another portal on a wall. Grab a box and drop it through the portal onto the turret.

Find another red X by looking down the hall and up. Shoot a portal there and return to cover (press the cover button to crouch behind walls or objects). Drop another box through the portal onto the turret.

After the turret is dealt with, jump through the portal and immediately go into the corner opposite the wall light.

Pop out around the corner to see the white wall behind the turrets and shoot a portal there. Return to the corner.

Run back down the hallway towards the first turret. When on the white floor, shoot a portal at your feet and go through. Pick up and drop both turrets. Go through the right door.

In the room with crates, look out the window to see a red X over the far turret. Go out the door, pop around the left door, shoot a portal at the X, and return to safety. Re-enter the right room.

Shoot the other portal at the wall and drop a box through.

To deal with the next turret, run to the other side of the left door, crouch, and walk slowly towards the door. Shoot a portal over the turret near the window of the other room (there is no X here). Run back to the right room and drop a box on its head.

For the next turret, run into the room and hug the window to the right. Pop out of cover around the corner and aim for the red X on the ceiling. Fire a portal, then retreat to the right room. Drop a box on its head, then run back into the left room.

Look towards where the last turret was standing, and slightly up to find it on the wall.

Stand towards the doorway you just came in from, grab a box, and put it on the button. When the door opens, a red laser will come out. Grab a box and hold it in front of yourself as a shield for the bullets. Crouch and move up against the grating, then drop the box. Aim for the ceiling above the turret, then aim at the ground. Drop through. Pick up and drop the turret.

Face where the turret was and look to the side opposite the grate. Hug the wall next to the grate and shoot a portal as close to the grating as you can on the other side. Shoot the other portal anywhere. Go through. Pick up and drop the turret. Walk towards the chamberlock.

Enter the elevator.

Run into the small room ahead. Pick up the Weighted Companion Cube and go into the next room.

Place the cube next to the black steps and use it to jump on top of the first step. Turn around, pick up the cube, and place it next to the next step. Use it to climb the next step. Face the next hallway, but do not go in.