Paul W. answered 04/23/19
Dedicated to Achieving Student Success in History, Government, Culture
As far as I understand it, the 'Fall and Rise' of urban settlements during the Middle Ages (476-1480) was, quite naturally, tied to the extent and scale of economic activity. As the Roman Empire declined, it was accompanied by a serious decline in trade and manufacturing. Backyard gardens aside, food is not grown inside of towns and cities; it has to be imported, from farms in the surrounding countryside and / or sources located farther away. The city of Rome, for instance, with, at its height, an unprecedented population of a million people, was dependent on grain shipments from Egypt. Small wonder, then, that as trade declined severely beginning in the 4th century (if not earlier), the populations - and, therefore the viability - of cities and towns within the Roman Empire also declined, resulting in some cases of their complete abandonment.
The precondition for the revival of urban settlements in Europe during the Middle Ages was a revival in trade. The consolidation of political control over the better part of Western, Central, and even portions of Eastern Europe by the Carolingian Empire in the 8th century saw a modest expansion of urban settlement. But, ironically, it was a force best known for destruction that did the most to jump start Medieval Europe's economy; the Vikings. If Vikings couldn't obtain the material goods - and slaves - that they desired through violence, they were perfectly willing to engage in trade. With their skills in handling their magnificent ships, the Vikings created a network of trade routes that stretched all the way from the Baghdad Caliphate to Iceland, including everywhere in between. They even contributed directly to the spread of urban settlements beyond the boundaries of the old Roman Empire, establishing the first towns in their homelands in Scandinavia, in Ireland, and in Russia. In Scandinavia, these originated as convenient locations for merchants to meet and engage in trade, often associated with religious cult centers. In Ireland, what began as fortified bases in which raiders could stay over during winter grew into permanent settlements engaged in manufacture and trade.
Elsewhere in Europe, cities that somehow had managed to survive the end of the Western half of the Roman Empire, such as Paris and Rome, began to experience growth. In the British Isles, the locations of former Roman cities that had been abandoned, such as London, York, and Chester, once again saw human habitation. As the saying goes, the three most important factors in business is 'Location, location, location.' In the 10th century, as a means of withstanding invading armies of Danish Vikings, the ruler of the English Kingdom of Wessex, Alfred the Great, oversaw the creation of a system of fortified towns called Burhs, each one located within a days travel of one or more other Burhs.
On the continent, among the most important factors for merchants was finding a relatively safe location to pitch their booths and engage in trade. As such, they usually chose locations such as outside the palace of a king, a powerful local noble, a bishop / archbishop (which were usually fortified and contained troops) or a monastery. Beginning roughly in the 11th century, as both wealth and population increased throughout Europe, merchants found that they could reside permanently in these locations and the resulting year round economic activity attracted craftspeople and others whose decision to settle in these locations led to the development of urban communities in the form of towns.
With the increase in the overall population of Europe during the period known as the High Middle Ages (1000-1300), the mainstream Medieval civilization expanded beyond the core regions of England, Western, and Central portions of the continent and, as this expansion took place, the conquerors / settlers established new towns. The Kings of England imposed their control over their Celtic neighbors, particularly Wales and Ireland, where control was consolidated with the building of fortified towns. The peoples of Central Europe pushed eastwards, into the lands of the Pagan Prussians and the Slavic people, establishing towns wherever they settled. See: Robert Bartlett's The Making of Europe: Conquest, Colonization and Cultural Change 950-1350. Princeton, New Jersey: Princeton University Press, 1993.