Not too long ago, I read a discussion on a Roman general's description of an abandoned walled city the he came across in his travels. He remarked on the impressive size of the wall itself, being both tall and spanning a great area. I wanted to read more about this, but can not recall the specific person, nor the city. Searches of Reddit and the open internet return results commensurate with my lack of detail. Can anyone help me with a name?
I think it’s possible you are thinking of Xenophon who was a Greek Mercenary Captain rather than a Roman General. He describes encountering several abandoned Assyrian cities in his work Anabasis which describes a large group of Greek mercenaries escaping the Persian empire after being on the losing side of the civil war in 401 BCE.
You can read his account of seeing these abandoned cities here
Source: Xenophon. Anabasis
You are likely misremembering: it was not a Roman general, but a Greek one, Xenophon, during the fateful march of the Greek mercenaries described in his later recollection Anabasis. The cities the Greeks encountered during their journey were Kahlu and Nineveh, which Xenophon mistakenly says were "inhabited by Medes." They were two of the formerly great cities of the Assyrians. You can read Xenophon's account here.
And you can learn more about this topic in a thread by /u/iphikrates over here.