It was in some audio lecture so I can't search it -- but I remember someone credible hypothesizing that the city of Rome hit a low of perhaps 12,000 people at some point? (Down from perhaps 800K - 1M?)
What was it like to live in the ruins and non-working infrastructure of a city built for hundreds of thousands more people? Why did the population number get so low?
Why did not some people settle there simply because there was a wealth of building materials? Were these residents just some skeleton-crew outpost of stone/brick/materials looters for other places?
Who has written/is writing about what Rome was like in its most abandoned state? Tangentially: I'm interested in reading more about the concept of people "living amidst the ruins" of an older, more-technologically advanced culture; which I understand to be fairly common in some places in the Middle Ages. Have there been any seminal, must-read works produced on this subject?
[I'm willing to read challenging (English Language) things, and have an adequate but unspectacular understanding of the high-points of first and second millennium European history... so I can hit the ground jogging, at least. I thank you for your time and for your expertise.]
Post-Empire, Rome ended up in a fairly consistent decline for centuries reaching it's nadir (where 12k inhabitants is a fairly valid estimate) in the 13th and 14th Centuries, particularly when the papacy was based in Avignon as the Pope had become, at that point, kept in power by the French alone who had been having issues with Sicily and the Holy Roman Empire, which had kept Italy in a state of flux. Rome naturally descended into anarchy during this time due to a vacuum of power but, once the papacy was restored in 1377, things improved.
Source The Popes - John Julius Norwich
Rome itself went into steep decline following the Gothic War. As other comments have mentioned, the urban areas began to recede, in this case, to an area focused around the Vatican hill. The best way to describe Rome during the period from 800 to say 1300 CE would be to imagine a number of small villages built in the ruins of the old city. The walls of the city mostly remained intact, but within those walls urbanization was replaced by overgrown vegetation and small agricultural plots interspersed between those small villages. This was a fate shared by Rome's sister to the east; when Suleiman finally entered Constantinople he found a 'city' of small villages with a somewhat larger urbanized cluster around the Palace in the extreme east of the city.