Two-sided walls (which appear as red lines in 2D mode) can be masked. Then each side of the wall can be edited.

In 3D mode, point the mouse cursor at either the floor below or the ceiling above any two-sided wall, and press M. A masked wall will appear. 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 in 3D mode (or Ctrl + H in 2D mode) to make a masked wall hittable by bullets and other projectiles. A hittable masked wall will appear thick in 2D mode.