The Mainz Merlon Mystery

[N._N.]

In other words: the text emphasizes that Mainz has Roman origins and still shows the signs of that. So why not put some "Roman" flair on the walls?

This is probably it, but I would take it a step further: The picture is not a stand-in for contemporary, 15th century Mainz, but rather for the city centuries ago. This matches the chronology with Mainz being listed in the 3rd World Age (while all other German cities are 5th or later) and the supposed Trojan and Roman history. No reader would expect an accurate depiction in that case. Seems kind of obvious to me now with the different 'Weltalter' that I wonder how I could have overlooked it at first...