An excellent description of Old Sarum
The modern city of Salisbury began its history two miles to the north of its present location, on top of the iron age hillfort known as Old Sarum. This impressive earthwork consists of an outer defensive wall and an inner rampart rising at an angle of over 45 degrees and measuring 40 feet from trough to top. The fortification, named Sorviodunum in Roman times, was occupied successively by the Romans, the Saxons, the Danes, and finally by the Norman conquerors of England.
Old Sarum was the site, in 1070, where William I disbanded his conquering army after having finally subdued the country four years after the invasion of 1066. The victorious troops were paid off in treasure, which the Normans had taken from the defeated Saxons. The construction of the royal castle (photo above), whose ruins are still visible today, had already begun under the direction of Osmund, the Conqueror's Chancellor (and, possibly, also his nephew). Around the castle and residence of King William I, a vibrant town was in the process of growing.
Under Lanfranc, the first Norman Archbishop of Canterbury, rural diocesan headquarters were ordered to relocate to major population centers. Accordingly, the See of Sherbourne was moved to Old Sarum in 1075. Herman, the Bishop who had moved over from Sherbourne, died in 1078.
It was left to his successor, the versatile Osmund, to build the new cathedral, which was consecrated in 1092. Just five days later, a great storm came and the building was largely destroyed by lightning. Reconstruction began in 1100, a year after Osmund's death. It was carried out under the new Chancellor, Roger, appointed by the newly crowned Henry I. In 1102, Roger was also named Bishop of Salisbury, and during his tenure, he accumulated great wealth and influence far beyond the boundaries of the town. Old Sarum reached the height of its authority and importance, at this time, also.
In 1139, four years after the death of Henry I, officers of King Stephen, who were distrustful of Roger's power, arrested him, seizing his estates and castles. Roger died later that year of fever at Old Sarum. The city of Old Sarum went into a decline and tension developed between the civil and religious authorities, escalating over the next 50 years. The solution to the tension, proposed by Bishop Herbert Poore to Richard I in 1194, was to move the cathedral away from Old Sarum to a virgin site on the banks of the nearby River Avon. Richard approved the move and the plans were drawn up. The townspeople actually began to move to the new site before any official announcement was made.