It would be facile to attempt to draw detailed conclusions from this inevitably superficial regional survey of the later Roman Empire. However, a number of points emerge with reasonable clarity. Firstly, the great cities of the Roman world were critically important for its economic development. The infrastructure which was necessary for feeding the huge populations of Rome and Constantinople had a transforming effect on the provinces of the empire. The delivery of grain, oil, wine, bacon, and dried goods in the form of annona deliveries both to Rome and Constantinople was certainly the largest enterprise in the transport of foodstuffs from rural producers to urban consumers ever achieved in the pre-industrial world. The agricultural economies of Africa and southern Italy, and of Egypt and southern Asia Minor were largely shaped by the demands of the capital cities. Major ports to handle the trade had already developed in the early empire, in particular Alexandria and Carthage, which grew in size and importance in late antiquity to become mammoth cities in their own right. The annona depended entirely on a secure and highly organized system of maritime transport, and shipping became the largest industry of the period. The effects of this are most obvious in the southern Asia Minor coastal region of Cilicia, which had always been an important timber-producing and shipbuilding region, but which now became one of the most highly developed and intensively urbanized parts of the empire, with its own major center at Corycus. The supply requirements of the city of Rome also had a transforming effect on the economy of southern Italy between the fourth and sixth centuries. It is fair to assume that much of the produce of southern Italy to Rome would also have been transported by sea.
Detailed analysis of commercial trading patterns suggests that they too were heavily influenced by the organization of the imperial annona. Ships that carried grain and other foodstuffs could carry additional cargoes of higher value for sale in urban markets. It is difficult otherwise to account for the domination of African fine pottery in the urban markets of the western Mediterranean. Egypt and Alexandria also became the main transit entrepôt for the imported luxuries of the East, including silks, gems, and spices, overtaking the overland routes through the Near East.
Meanwhile the most important overland lines of communication in the Roman Empire were interrupted or broken. The Roman roads that ran though the Balkans, which had held the eastern and western parts of the empire together until the late fourth century, became insecure and were for long periods simply given up to the control of the Goths, Huns, Avars, and other barbarian groups. Insecurity in fifth-century Gaul led Rome to lose control over the great north–south trunk road from Marseilles and Arles to the northern capital at Trier, which was abandoned as an administrative center. Thus the northwest provinces, which depended on secure overland communications, were also lost to the empire. The result of these developments was a Roman world which was much more strongly oriented towards the Mediterranean than it had been in the age of Augustus and the early empire. The key centers of Roman economic and political power were now its sea ports.126 Constantinople, the new capital, was above all a maritime city.
Urban settlements, that is cities, were a central defining feature of the Roman Empire. The cities were the locations where large landowners, the members of the ruling elites, could concentrate and display their wealth. They thus became the inevitable foci of power at a regional or provincial level. From the perspective of the imperial authorities it was also important that tax collection was organized on a city-by-city basis, even though the majority of tax income was levied on landed property. Thus state and regional interest converged in the creation and maintenance of prosperous and sustainable civic institutions, which became the visible and material symbols of the empire itself. The viability of the empire may reasonably be measured by the presence or absence of city life. This criterion provides one of the most valuable distinctions that can be drawn between the western and eastern halves of the empire in late antiquity. In Britain, northern Gaul, and most of the Danubian provinces, urban settlements shrunk in size or disappeared during the fifth century. There is no evidence for civic buildings, local governmental structures, or large-scale communal organization outside that of the church. The populations of these regions were barely linked to wider networks of commercial exchange. They now lived in villages or other small rural settlements, which exchanged goods and services within a localized regional economy, but had few connections to a wider network of exchange. Their links to the Roman Empire had effectively been broken in social and economic, as well as in political, terms.
This pattern of settlement contrasts acutely with the situation that can be observed in the eastern empire. Here cities continued to flourish at least until the middle of the sixth century, and in some regions much later. This was not at the expense of smaller rural settlements, but as a consequence of the age-old symbiosis of town and country, which was the bedrock of classical city-state existence. Thus regions with flourishing small cities, such as the harbor towns of Lycia in southwest Asia Minor, were supported by a correspondingly prosperous highland hinterland. The densely populated limestone massif of northern Syria, covered with villages large and small, had its exact counterpart in the huge cities of late Roman Syria, Cyrrhus, Apamea, and above all Antioch.
Global security was vitally important for sustaining civic life in the Roman provinces. The greatest single threat to growth and sustainable prosperity, at least until the outbreak of the great plague, was not epidemic disease, blight, or famine due to natural causes, but warfare. The Roman Empire was sustained, in the last analysis, by its ability to deploy military force effectively to protect its communities. When the use or the threat of force failed to achieve this aim, as in fifth-century Gaul, in the central Balkans against Attila's Huns, or in Illyricum as a whole against the Avars and Sclaveni in the later sixth century, then the provinces slid from Rome's grasp and civic life collapsed. The extraordinarily high levels of urban prosperity and the large populations of the eastern empire were only threatened when the Roman state was no longer able to deploy its military force and diplomatic skills to protect them.