Here you are going down the elevator. At the bottom, duck and get out. Shoot the alien in front and head left.
Turn the door and shoot the pig. Go to your right and see the door with buttons to its left. Push them so that they say 1-0-1. Now go in the door.
Go through the hall and keep left; it's dark. When you get to the elevator, push it to go up.
Turn right and go towards the closer wall. Push the part in the middle, and get the blue key. Kick the glass and go out of the window.
Keep going through the street and go up to where the wall says "3D Realms Demolition Co." Go through the way right and turn to see the blue access. When you put the key in you will see a lock combination. Push the buttons so that they say 0-1-1-0. There will be another switch so push it. Now the building in front will fall down.
When it is lowered all the way, go up to it. In the corner to the right, you will find the yellow key.
Now go back to the demolition area, you will find the place for the yellow key. Open the door and turn left. Bust the vent in front and go through. At the end of this duct, bust the next vent, then drop out.
Go up to a urinal and Duke says, "Ah, much better!" Go up to the mirror and listen to Duke say, "Dan, I'm looking good!"
Once out of the bathroom, go up to the phone and call for help. Sorry, but the phone just beeps back at you!
Turn right and then turn left. Go to your first right. When you see the counters, go through them and open one in the middle on the left. Now you can get the red key.
Go back out of this area and turn around right. Soon you will see a door where you can put the red key in.
Once you put the key in, go in and look right. You will see a vent. You have to jump up to it, then bust it. Go through the duct and bust the vent at the other end.
Drop and go to the other end and push the switch.
Go back through the curtain and jump on the new circle. Then jump onto the higher one. Go through the hall and when you see the end don't go to it. You'll be trapped and the screen says "We're gonna fry your ass, Nukem!"