Vision is settled by height bands rather than by a line traced between two players. Stand above someone and you see them; stand in the pit and the plateau above you is dark.
Fog of war in Field of Valour does not ask whether an unbroken line runs between two heroes. It asks which height band each of you is standing in, and which band anything between you rises to.
The world is cut into bands. Your own band is the one your feet are in. A cell of the map is opaque to you when whatever occupies it - the ground itself, or a blocker standing on it - reaches a band above yours. That single comparison produces the behaviour the genre expects: high ground sees low ground, low ground does not see high ground, and a wall, a cliff or a treeline hides whatever stands behind it at its own height rather than whatever a ray happens to clip.
The consequence is that elevation is information you can carry. Walking up the stairs is not a movement decision that happens to improve your view. It is the view.
The rule is symmetric where it should be and asymmetric where it should be. Two heroes on the same band see each other or neither does. A hero on the plateau sees a hero in the pit below and is not seen back, until the one in the pit climbs.