Throughout western Europe, the effect of the Germanic invasions which completed the decline of the Roman Empire was to destroy the Roman municipal organisation. After the Anglo-Saxon settlement of Britain, the ruins of Roman colonies and camps were used by the early English to form tribal strongholds. Despite their location, burhs on the sites of Roman colonies show no continuity with Roman municipal organisation, and instead resemble the parallel revival of urban centres in continental Europe. The resettlement of the Roman Durovernum under the name "burh of the men of Kent," Cant-wara-byrig or Canterbury, illustrates this point. The burh of the men of West Kent was Hrofesceaster (Durobrivae), Rochester, and many other ceasters mark the existence of a Roman camp occupied by an early English burh. The tribal burh was protected by an earthen wall, and a general obligation to build and maintain burhs at the royal command was enforced by Anglo-Saxon law.
Offences in disturbance of the peace of the burh were punished by higher fines than breaches of the peace of the hām or ordinary dwelling. However, neither in the early English language nor in the contemporary Latin was there any fixed usage differentiating the various words descriptive of the several forms of human settlement, and the fortified communal refuges cannot accordingly be clearly distinguished from villages or the strongholds of individuals by any purely nomenclative test.
At the end of the 9th century and beginning of the 10th century there is evidence of a systematic "timbering" of new burhs, with the object of providing strongholds for the defence of Wessex against the Danes, and it appears that the surrounding districts were charged with their maintenance. It is not until after the Danish invasions that it becomes easier to draw a distinction between the burhs that served as military strongholds for national defence and the royal vills which served no such purpose. Some of the royal vills eventually entered the class of boroughs, but by another route, and for the present the private stronghold and the royal dwelling may be neglected. It was the public stronghold and the administrative centre of a dependent district which was the source of the main features peculiar to the borough. Many causes tended to create peculiar conditions in the boroughs built for national defence. They were placed where artificial defence was most needed, at the junction of roads, in the plains, on the rivers, at the centres naturally marked out for trade, seldom where hills or marshes formed a sufficient natural defence. Typically, the fortification of a burh consisted of earth ramparts faced with timber. Palisades were sometimes used.
The solution that Alfred devised for this apparently intractable predicament was nothing short of a revolution and that revolution began now in the 880s. If the Vikings could attack anywhere at any time, then the West Saxons had to be able to defend everywhere all the time. To make this possible Alfred ordered the construction of a network of defended centres across his kingdom, some built on refortified Roman and Iron Age sites, some built completely from scratch. These burhs were to be distributed so that no West Saxon was more than twenty or so miles – a day's march – from one of them.