If a straight line can be drawn between the center of the caster’s space and the center of the target space, without crossing a wall, closed door, or terrain, the spell can be used. Otherwise, it is blocked. LOS can pass through an open door, and it can pass through the openings between sectors. It is blocked by terrain, but not by dropped objects, and not by wizards.

To create or destroy a wall, which is not “in” a square, but on the line between squares, you must have LOS to any point on the wall.