A One-Sided Masked Wall is only visible from one side. It is usually referred to as a "One-Sided Wall".
In 3D mode, point the mouse cursor at the floor just below any two-sided wall, and press 1
.
A One-Sided Masked Wall will appear. These are useful for hiding secret
rooms. The only difference between a Masked Wall and a One-Sided Wall
(aside from the obvious fact that one side is invisible) is that
One-Sided Walls cannot be made transparent.