could assemble and arrive. Although the landowners were obliged to the king to supply these men when called, during the attacks in 878 many of them abandoned their king and collaborated with Guthrum. With these lessons in mind Alfred capitalised on the relatively peaceful years following his victory at Edington with an ambitious restructuring of Saxon defences. On a trip to Rome Alfred had stayed with Charles the Bald, and it is possible that he may have studied how the Carolingian kings had dealt with Viking raiders. Learning from their experiences he was able to establish a system of taxation and defence for Wessex. There had been a system of fortifications in pre-Viking Mercia that may have been an influence. When the Viking raids resumed in 892 Alfred was better prepared to confront them with a standing, mobile field army, a network of garrisons and a small fleet of ships navigating the rivers and estuaries. Administration and taxation Tenants in Anglo-Saxon England had a threefold obligation based on their landholding: the so-called "common burdens" of military service, fortress work, and bridge repair. This threefold obligation has traditionally been called trinoda necessitas or trimoda necessitas. The Old English name for the fine due for neglecting military service was . To maintain the burhs, and to reorganise the fyrd as a standing army, Alfred expanded the tax and conscription system based on the productivity of a tenant's landholding. The hide was the basic unit of the system on which the tenant's public obligations were assessed. A hide is thought to represent the amount of land required to support one family. The hide differed in size according to the value and resources of the land and the landowner would have to provide service based on how many hides he owned. Burghal system The foundation of Alfred's new military defence system was a network of burhs, distributed at tactical points throughout the kingdom. There were thirty-three burhs, about apart, enabling the military to confront attacks anywhere in the kingdom within a day. Alfred's burhs (of which 22 developed into boroughs) ranged from former Roman towns, such as Winchester, where the stone walls were repaired and ditches added, to massive earthen walls surrounded by wide ditches, probably reinforced with wooden revetments and palisades, such as at Burpham in West Sussex. The size of the burhs ranged from tiny outposts such as Pilton in Devon, to