My textbook claims only a few towns existed north of the Alps in Europe. This makes me wonder what Rome looked like, because I thought it was almost completely deserted after the fall of the Roman Empire. Were houses destroyed, left empty, or was there way more life in the city in 800?
I go into this in depth in this earlier post, albeit from the perspective of 700 CE rather than 800 CE.
Basically, all the main late antiquity buildings are structurally intact, though emptied and abandoned (think Detroit), excepting those that were converted to churches. You have wooden housing for the bulk of the population, probably utilizing existing Roman ruins as part of the walls or foundations, and a few two story stone/spoila houses for the aristocrats. There's a lot of pastureland inside the Aurelian walls. Also a lot of mosquitoes. Local city politics is apparently still quite lively and factional. There was some sporadic artisanal production, as well as a steady stream of pilgrims coming to see the tombs and holy sites.
Though the city is not as big as it was in Roman times, it was one of the largest cities of western Europe at the time.
The first thing to consider is that your textbook is, if I daresay, wrong. Of course, in the “barbarian West,” towns were not the important centres they used to be, for many reasons. However, each bishopric needed to have a urban centre to function; and this urban centre was alive, even though its population had dwindled since the good ol' times of the Empire. It was still in some regards a trade hub; it seems that the count (in the Frankish kingdoms), just like the bishop, was sometimes living in the city; and the new popularity of shrines and relics helped drawing new forms of activity towards the cities that could boast the patronage of an important saint. The functions of the civitas had changed (its central administrative role, most notably, had almost vanished), but urban life was continuous over the Early Middle Ages. The exception to this rule may be sub-Roman Britain, in which it seems that the urban culture experienced a sudden collapse in the 5th-6th century.
As for Rome, I cannot be very precise, but it is clear that the area enclosed by the walls was enormous by medieval and ancient standards, too vast for the remaining population (which had suffered, among other ills, from the Byzantine reconquest of Italy in the 6th century: re-iterated sieges of Rome had almost emptied it from its population at some point, as Procopius tells us in the first book of his Gothic War). Therefore, besides “normal” habitations, Charlemagne would have seen patches of cultivated land inside the space of the walls, crumbling buildings (basically everything that was related to the “civic” culture of ancient Rome: (amphi)theatres, fora, perhaps a few pagan temples [1]…) and a vivid ecclesiastical core, around the bishop of Rome and his familia (his court would not have been very different from that of other bishops in similar cities, at that time).
[1] Procopius tells us that the temple of Janus was still standing in the 6th century; however, it is possible that by the end of the 7th, it had been destroyed, or recycled.