No text, but a visual code

[Linda]

   As I previously mentioned, the author sought to suggest the Earth's sphericity through a metaphorical device—such as those high walls and the houses clinging to the cliffs.

Yes, you said it before, but I still don't understand how houses on cliffs are a metaphor for the earth being a sphere. 

 Some maintain that it is a map, and various locations on Earth are proposed. I believe this stems from a misinterpretation of the drawing—which depicts a high wall, what appear to be cliffs, and small houses clinging to the faces of those cliffs. While this seems to describe real places, it is actually a metaphor intended to convey the idea of Earth as a sphere suspended in space, just like the other spheres. The entire image is a representation of the skies.