Getting the Yellow Key / Getting to the Yellow Key Slot (4)

Enter the Generator Room, which was previously locked by the Red Key. An Alien Drone (an enemy type) will greet you, with a Trooper and a Captain below.

Approaching the generators at the base of the room will alert three more Alien Drones.

On the right side of the generators, find a Shotgun. In a crevice at the front, there's Microwave Expander Ammo. Another Chaingun Cannon is on the right side, and an Atomic Health (a health pickup) is located between the generators.

On the other side, an Alien Drone will attack, alongside Microwave Expander Ammo opposite the first one, and a Large Medikit (a health pickup) in the corner.

A Battlelord (a powerful enemy type) will also notice you from the room across from the generators, where another Alien Drone is present.

After defeating these enemies, you can collect the Yellow Key from on top of a box. Two Pipebombs and Body Armor are slightly higher.

Behind crates, in the fire, another Alien Drone guards an Atomic Health.

Return to the Catwalk area and use the Yellow Key on the Yellow Key door to access the Crane Room.

Inside the Crane Room, an Alien Drone will drop from the ceiling.

Further into the room, a Commander will attack from above, joined by two to four more Alien Drones.

Near the entrance, you'll find two Pistol Ammo pickups.

On the left side of the room, there's a box of Shotgun Shells and a Large Medikit.

In the center of the room are two RPG Ammo pickups and an Atomic Health.

Proceed to the right side of the room and take the conveyor belt. Here, you'll encounter a Trooper and two Commanders. Shotgun Shells are in a corner.

SECRET 5: Climb the boxes to the highest point. A Trooper and a Commander will attack from a hole in the boxes. Drop into this gap after defeating them to find an RPG and Protective Boots.