Its a strange crater..
Discussion
It was initially considered to be an impact structure (as is clearly the case with the other three), but a closer study in the 1950s to 1960s suggested that it might instead have been formed by terrestrial processes.
Yes, likely a very ancient volcano
But it's easy to see how this lent itself to building a city based on the different rock types, according to Plato's retelling, concentric circular canals, maybe 3 rings?
I think the center has all those bitcoin powered businesses in it that we keep hearing about but can't seem to find.