Commonly referred to as the "Doom Door". This door rises into the ceiling when opened, and lowers to the floor when closed.
Create the door sector and tag it
Enter 3D mode and texture the door's frame. You may need to flip the textures around with
F to make them look proper. Press
O on each wall of the door frame to orientate them to the floor (this forces the frame to stay motionless while the door opens and closes). Place an
Speed[0,Speed], S[Delay,10]) in the door sector. I recommend
Point your mouse cursor at the ceiling of the door sector. Hold the
Left Mouse Button to 'lock' the ceiling, and press
PGDN until the ceiling touches the floor. Texture your door, and use the
8 keys to adjust the texture repeats so they fit properly. Now your door is complete. In the game, you can stand in front of it and press the action button to open and close it.