Visual assessment:
(1) AO should be subtle but noticeable if you know what to look for.
(2) In a terrain that includes ambient light as well as sunlight, such as Belgian Bog, direct light shadows should look slightly bluish, as the ambient light they get comes mostly from blue sky scatter, which favors blue (and green to a lesser extent). This can now be visually verified.
(3) Looking at the saturated white terrain where Acropolis Bay begins (original civic center placement), now you will see a ground covered with white stone tiles which wasn't even visible before. Other terrains with near white stone tile paths are now much easier to appreciate, and less blinding, such as Belgian Bog and Alpine Valley. The problem with this approach, however, is that the problem is likely to remain if the shader deals with it. The art should be fixed, and these changes removed; which is why I call it "temporary" in the title.
The diff includes a fix of the Ptol civic centre. This change can be removed; it is not too important; I changed the model to hide two graphics problems that it triggered, namely flickering shadows on the terraces, and light leaking under the shadow casting walls.