It seems you can't talk about post-Roman Western Europe without causing a debate about the actual extent of ruralization.

My view is that it is that across the board, the extent of destruction by the evil bearded barbarians is exaggerated because of a few very specific, very violent incidents (justifiably) mentioned by chroniclers, leading to the longstanding belief that destruction was widespread. This is backed by all sorts of evidence from across Europe, from Linguist John McWhorter who disproves chronicles of Anglo-Saxons "overrunning" the British Isles by citing a slow and incomplete shift from Celtic to Germanic based language, to the increase of North African Red Slip Pottery found in the Northern Mediterranean in the fifth and sixth centuries, when the Germanic Vandals had founded their own kingdom in the old Roman Province of Africa Proconsularis (modern-day Tunisia and the Lybian coast).

Of course, there is no denying that late antiquity was not a happy time for the Western Roman Empire. The very same Vandals, for example, set out from Africa and sacked Rome in 455. However, this sack wasn't some mindless rampage: Emperor Valentinian had almost brought the Vandals (and North Africa) back into the fold of the Empire (North Africa continued a very wealthy, fertile, and consequentially desirable). However, Valentinian was killed by a usurper, Petronius Maximus, who then cancelled various agreements with the Vandals. There is debate as to whether the sack was less of a sack and more "killing that asshole Petronius and taking the nice stuff from temples".

Another common misconception also involves the nature of the Empire's relation with many "Barbarians". Towards the late Empire, large portions of the Roman Army was made up of ethnic "Barbarians". The concept of Roman citizenship was always more tied to enfranchisement rather than actually being from Rome, meaning that as central and eastern European peoples migrated into the empire, a large number of them were not only conscripted into the army, but many could be considered "Romanized". Indeed, Flavius Odoacer, infamous for having "ended" the Empire, was arguably more roman than Romulus Augustulus whom he deposed; we have no indication of Odoacer's non-roman heritage, while we explicitly know that Romulus Augustulus' father was not Roman. Further, Odoacer deposed Augustulus at the head of the Roman army where he was the uncontested leader for some time.

Population displacement can work in unexpected ways. In northeast Italy, for example, the great city of Aquileia, home to a Catholic Patriarch (super boss-man of religion) after countless sacks was nearly entirely depopulated by the end of the sixth century, and would not be rebuilt for some four hundred years (and would never recover). Some people might have taken to the countryside, sure, but the Patriarch and many citizens relocated to Grado, a town by the sea separated from the mainland by a lagoon, reachable only by crossing a narrow causeway over the lagoon's marshy edge: an ideal place to defend, nearly impossible to surround, and easily supplied by the sea. Incidentally, Grado's lagoon was also a very comfortable haven for ships, making it an ideal stopping point for merchants, as well as a useful naval lifeline to the Byzantine Empire.

Although the Lombard Kingdom of Italy would be characterized by near-constant conflict with the Byzantines over hegemony in the peninsula, the Lombards claimed legitimacy by imitating, as best they could, Roman urban life. For example, although the King's court was de facto in the dependable fortress of Pavia, the nominal capital remained the great (albeit depopulated) Roman City of Mediolanum (modern Milan). Indeed, the heart of Milanese finance (Italy's Wall Street, if you will) is a circular square called Piazza Cordusio, a curuption of Curia Ducis, or "Ruler's Court". You can imagine what building one stood in the place of the modern Piazza.

While some cities would see their population fall, other cities would grow. Ravenna, the historic base of the Imperial Fleet, had already housed the Imperial Court when Rome was threatened; at the time, Ravenna was found on an elevated spot surrounded by a wide brackish floodplain, ideal for unloading troops and supplies from ships. As the Byzantine Empire attempted to take Italy back from the "Barbarians", Ravenna was an ideal base of operations, and would become the capital of the Eponymous Exarchate (overseas military province).

As the Byzantines fought tooth and nail to keep Italy, Ravenna and Grado weren't the only important bases of operations. South of Ravenna, the five other maritime cities that flourished were Rimini, conveniently located within the two branches of an a small river delta, Pesaro, flanked by two easily defensible hills, Fano, also found on a hill, and Senigallia, found on a hill almost completely hugged by a river. Together, these cities were called the "Pentapolis". All benefited from wide, sandy beaches stretching for miles on either side (perfect for unloading ships) and some sort of defensive advantage (very important in the very very flat northern Italian plain). So ingrained was Byzantine presence in the Pentapolis, that the region is still called "Romagna" to this day, as the Byzantines were the last "Romans" to leave.

And of course, who can forget the lagoon north of Ravenna, south of Grado? Fed by the Brenta River in the south and the Sile River in the north, the communities of Chioggia, Malamocco, Torcello, Murano, and countless other sprang up in the sheltered islands as early as the sixth century. With the advantage of the nearby mouth of the River Po connecting this lagoon to the whole of Northern Italy, the inhabitants would become a major shipping hub, Italy's last line to Byzantium long after the Pentapolis had fallen. These communities would go so far as to stipulate rules for their self-governance, electing a leader and creating organs of government. The chosen meeting place, originally a bank of sediment deposited by the current of the Brenta River, was called the Rio Alto, literally, "High Bank", in the local Latin Vulgate. Soon, the "Rialto" and its nearby islands would grow, as marshy land was filled in and constructed on. But that is a longer story than I have time to tell, plus John Julius Norwich did it better than I ever could in "A History of Venice" (Penguin, UK; 1989), definitely worth checking out.