Making a door
Making a door is a simple process, but may also be one of the first obstacles a person new to Doom mapping runs into.
Suppose that we have a map similar to the one shown on the diagram to the right, with two rooms connected by a hallway. Sector A is a first room, Sector C is the second room, and Sector B will be our door.
Start off by lowering the ceiling height of Sector B to be equal to that of the floor. For example, if your floor height is zero, make the ceiling height zero as well. Make sure to apply upper textures where needed.
Next, select the linedefs facing towards the two rooms (Sector A and C), and give them a Line Type of 1 (DR Door Open Wait Close). Make sure that your door's linedefs are facing outwards towards the two rooms. If you do not do this, there is a good chance the door will not function.
You should now have a door that, when being triggered, raises up to the ceiling, waits a few seconds, and lowers back down. But there is one more thing that should be done for aesthetic quality. Select the doorjamb linedefs on both sides of the door (highlighted in red), and give them the Lower-Unpegged flag. This will prevent the texture from moving up/down as the door is in motion.
When a door opens, it will move up to the height of the lowest neighboring ceiling minus 4. If a door fails to open normally, verify that the door sector does not have any neighboring sector with a ceiling too low, and remember that sectors can be discontinuous. An example would be trying to put little decorative alcoves inside the door tracks, or having two different doors that touch each other: these will result in malfunctioning doors. It is even possible for such a malformed door to sink into the ground: if the lowest adjacent ceiling is at a height lower than the door sector's floor + 4, the door will instantly move down, instead of up.