The hilltop at Old Sarum lies near the Neolithic sites of Stonehenge and Avebury and shows some signs of early settlement. It commanded a salient between the River Bourne and the Hampshire Avon, near a crossroads of several early trade routes. During the Iron Age, sometime between 600 and 300 BC, a hillfort (oppidum) was constructed around it. The Romans may have occupied the site or left it in the hands of an allied tribe.
At the time of the Saxon invasions, Old Sarum fell to King Cynric of Wessex in 552. Preferring settlements in bottomland, such as nearby Wilton, the Saxons largely ignored Old Sarum until the Viking invasions led King Alfred (King of Wessex from 871 to 899) to restore it's fortifications. Along with Wilton, however, it was abandoned by its residents to be sacked and burned by the Dano-Norwegian King Sweyn Forkbeard in 1003. It subsequently became the site of Wilton's mint. Following the Norman invasion of 1066, a motte-and-bailey castle was constructed by 1070. The castle was held directly by the Norman kings; it's castellan was generally also the sheriff of Wiltshire.
In 1075 the Council of London established Hermanas the first bishop of Salisbury, uniting his former sees of Sherborne and Ramsbury into a single diocese which covered the counties of Dorset, Wiltshire, and Berkshire. In 1055, Herman had planned to move his seat to Malmesbury, but its monks and Earl Godwin objected. Herman and his successor, Saint Osmund, began the construction of the first Salisbury cathedral, though neither lived to see its completion in 1092. Osmund served as Lord Chancellor of England (in office c. 1070–1078); he was responsible for the codification of the Sarum Rite, the compilation of the Domesday Book, which was probably presented to William at Old Sarum, and, after centuries of advocacy from Salisbury's bishops, was finally canonised by Pope Callixtus III in 1457.
The cathedral was consecrated on 5 April 1092 but suffered extensive damage in a storm, traditionally said to have occurred only five days later. Bishop Roger was a close ally of Henry I (reigned 1100–1135): he served as viceroy during the king's absence in Normandy and directed, along with his extended family, the royal administration and exchequer. He refurbished and expanded Old Sarum's cathedral in the 1110s and began work on a royal palace during the 1130s, prior to his arrest by Henry's successor, Stephen. After this arrest, the castle at Old Sarum was allowed to fall into disrepair, but the sheriff and castellan continued to administer the area under the king's authority.