– Green (Left): The green encircled wall is a door as it’s the middle wall of a 3-wall/monolith line.

– Red: NOT a door. The 3 walls are not in a straight line.

– Green (Right): 4 walls/monoliths in a line. The green encircled wall is a door. However. the red encircled monolith is NOT a door. A monolith can never be a door.