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 concept of a network of burhs as a defence in depth is usually attributed to Alfred. This network is described in a manuscript document which has survived in later iterations, named by scholars the Burghal Hidage, which lists thirty three burhs in Wessex and English Mercia. Most of these survived into the post Norman Conquest era and are the core of later Parliamentary Boroughs and municipal corporations. Commercial significance Following the successful reconquest from the Vikings by Alfred's descendants Edward and Æthelstan, the latter made a series of reforms in law, the Codes issued at the Council of Grately, which gave additional impetus to the urban development of the burhs which hitherto had been mainly forts. The burhs drew commerce by every channel; the camp and the palace, the administrative centre, the ecclesiastical centre (for the mother-church of the state was placed in its chief burh), all looked to the market for their maintenance. The burh was provided by law with a mint and royal moneyers and exchangers, with an authorised scale for weights and measures. Mercantile transactions in the burhs or ports, as they were called when their commercial rather than their military importance was accentuated, were placed by law under special legal privileges in order no doubt to secure the king's hold upon his toll. Over the burh or port was set a reeve, a royal officer answerable to the king for his dues from the burh, his rents for lands and houses, his customs on commerce, his share of the profits from judicial fines.