Used by the routine at DRAW_CELLS.

For a cell behind the knight -- the five the drawing order (DRAW_CELLS) draws walls for, one less in U or one more in V -- the building on it is drawn as two walls standing on its near edges: the definition for the cell's type and the view (BUILDING_TABLE, type times two plus VIEW) gives each face eight columns of a lower and an upper tile, and DRAW_WALL_COLUMN draws each column from the cell's corner (PROJECT_CELL) rightwards, down the screen along the first face and then, mirrored, up it along the second.

Walls are drawn nearest first among themselves, and each claims the screen columns it covers (COLUMNS_DRAWN): a tile is written over what is under it, and a wall and its colour reach from its bottom to the top of the play area, so nothing behind it in the same column could show. A wall further back only fills the columns still unclaimed, and as soon as all sixteen are claimed the routine stops. The drawing order puts the things in an open cell after the walls that stand behind it and before those in front of it, which cover them.