This is one gloomy and cold opening to a gloomy and cold episode.

When entering the main hallway, locate the health powerups if you want to stay alive fighting the mutants.
There is a machine gun behind a well at the far end of the hallway. Push the wall beside it.
The Gold Key is found in the major block behind another pushwall.
If you get into the secret room with columns you will see what you will think is the elevator to the secret level. But how to get to it?
The trick is that what appears to be the ordinary elevator is a pushwall. The elevator you have seen is the one that leads to floor 2.
To get to the secret floor you have to be involved in even more heavy fighting with both mutants and SS past a pushwall north of the elevator.
Finally you find yourself in the north-western corner of the floor. Push the nearby wall to get the chaingun and the 1-up, and go through another dummy elevator. On your way to the secret elevator you may fight it out with a Grosse brother.