The big bank and ditch encloses 56 acres.
It was an Iron Age hillfort, and then a Roman settlement. A Roman road named the Portway ran from here to Silchester.
The site was reoccupied in the 10th century, and developed into an important town, with a cathedral and a Royal castle built after the Norman Conquest.
In the 13th century, Bishop Richard de Poore decided to resite the cathedral in the valley below, where water was more plentiful. This became the major cathedral city of Salisbury.
Old Sarum, although depopulated, contined to return two members of Parliament until the 1832 Reform Bill.
Old Sarum is now in the care of English Heritage.