It's been many years since I've done much math, but it seem to me that
the visibility of any point on the wall from any point on the ground
could be computed with an integral over the distance, with a function
taking into account the variable density of the air at a particular
point on line from viewer to target.  The top of the walls should come
into view quite a while before the bottoms.

(Vague stirring of bits of calculus in my mind.  Wondering if I still
have the calculations for the volume of that ellipsoid Traveller ship
floating around anywhere...)

--
Bicycles are the most efficient machine yet invented for turning energy
into motion. Indeed, the bicycle has been accurately described as a kind
of green car, which can run on tap water and tea cakes and, moreover,
has a built-in gym.

Lord Taverne, UK House of Lords, in Nov 2015 debate