Walls that appear as a two-sided wall (red line) in BUILD can be masked. Then both sides of the wall can be edited.
Enter 3D mode and point the mouse cursor at the floor just below any two-sided wall (two-sided walls are displayed as red lines in 2D mode). Press M
, and a visible two-sided wall will appear where the red line is. Press B
(in either 2D mode or 3D mode) on the Masked Wall to make both sides of it blocked, so the player can't walk through it. A blocked Masked Wall will appear pink in 2D mode. Press H
(Ctrl+H
in 2D mode) to make a Masked Wall hittable by bullets and other objects. A hittable Masked Wall will appear thick in 2D mode.