Force fields are another well-known trick in Wolf3D. If you place a "deaf guard" floor code (006A) adjacent to both sides of a door, the door will be "invisible" (effectively a "force field") -- because door tiles are assigned the floor codes they're adjacent to, and the tilemap value is set to 0 (empty space) with any tile with that floor code.
There are a number of different things you can do with force fields, including using them to block secret areas and using them like static objects (note that the player and enemies can shoot through them, and enemies can "open them" (remember, they're really doors.))