Plan a 36″ x 24″ table (6x4 of your 6″ floor tiles) using your actual terrain pieces. Click a tile’s centre square to place a column, and click any edge square to cycle it between open, wall and door. Corners always stay clear.
Rules this tool enforces: a column can only sit in a tile’s centre square, and a wall or door can only be placed on an edge square — columns and edge pieces are placed independently of each other. Corner squares can never hold anything.