Anglo-Saxon fortified settlements, built mostly in the late 9th and early 10th centuries as a response to Viking attacks. The first burhs were built at the end of the 8th century in the Midland kingdom of Mercia, but their systematic use as defensive centres began in Wessex under Alfred the Great (r. 871-899). In the 880s Alfred established a network of 30 burhs across Wessex so that no part of the kingdom was more than 32 kilometres from one. There were two categories of burh. The largest were planned as permanent settlements and market centres, most of which developed into successful towns. There was also a catagory of smaller burhs that were intended only as temporary forts: most of these were probably abandoned by the mid 10th century. Some burhs, such as South Cadbury, were sited to take advantage of surviving iron age fortifications; others such as Bath, Chichester, Exeter and Winchester, made use of old Roman fortifications. Burhs such as Walingford and Wareham had defences that were modelled on Roman forts; many others utilized natural defences. Each burh was given a tax assessment and a garrison of peasant levies according to length of its walls. The formula recorded in the Burghal Hidage, compiled c914-18, was that four men were needed to man each pole (about 5 metres) of wall, and that one man should be supplied by each hide (the area of land needed to support one peasant family). Thus Wareham, whose defences were 400 poles (about 2,000 metres) long was assigned 1,600 hides of land. The defences were usually built of clay or turf, topped with a timber palisade, though in a few cases these ramparts were rebuilt in stone in the 10th century. Alfred's son and successor, Edward the Elder (r. 899-924) and daughter Aethelflaed, lady of the Mercians, methodically extended the system of burhs to consolidate the conquest of the Danelaw.
Notes from Encyclopaedia of the Viking Age by John Haywood. Thames
& Hudson 2000. pp 38-39
Additional information about burhs - Alfred the Great and the burhs
also Burghal Hidage (Alfred's towns, the burhs)
and Alfred the Great, including information on the Burghal Hidage