Nördlingen, in Bavarian Swabia, is like a crater inside a crater, as on the moon.
Like Rothenburg and Dinkelsbühl, it has a still-unbroken city wall. (The other two, also in Bavaria, are not round.) Unlike them, it lies at the centre of a depression, 25 km in diameter, the Nördlinger Ries, caused by a prehistoric meteorite.
From a 1967 book edited by Toynbee
Google Earth; an industrial area has attached itself to the town like a crystal
Edge of the Nördlinger Ries
Editor, Cities of Destiny, Thames & Hudson, 1967