“Of this city of Rome you could not say either that it was left unfortified with a Lacedaemonian bravado or that it was enclosed in fortifications of a Babylonian magnificence. … You have not, however, you Romans, neglected to build walls; only you have run them round your empire and not round your city. You have placed them in the uttermost parts of the Earth; yet they are magnificent walls which are worthy of you and are a sight for the eyes of all who live within their shelter – though it would take an intending sight-seer months or even years to reach them if Rome itself were the starting-point of his journey; for you have pushed your way beyond the outermost circuit of the Inhabited World and there, in no-man’s-land, you have drawn a second circuit with a more convenient tracée which is easier to defend – for all the world as though you were simply fortifying a city. … This circuit is utterly impregnable and indestructible at every point; it outshines all others; and no system of fortifications that was ever constructed before bears any resemblance to it.” [Footnote: Aristeides, P. Aelius: In Romam, edited by Keil, B., in Aelii Aristidis Quae Supersunt Omnia, vol. ii (Berlin 1898, Weidmann), pp. 114-15 (Or. XXVI, §§ 79-84).]
Dura-Europos was founded in 303 BC by the Seleucids. It controlled the river-crossing on the route between the other newly-founded cities of Antioch and Seleucia (which was further east, on the Tigris). In the later 2nd century BC it came under Arsacid Parthian control. The Romans captured it in 165, in the reign of Marcus Aurelius, roughly at the time Aristides was writing, and abandoned it after a Sasanian siege in 256-7 (the Persians may have used poisonous gases). It was then covered by sand and mud. American archaeologists discovered it in the late nineteenth century.
A Study of History, Vol VII, OUP, 1954