The Abbey was probably a Celtic foundation of c600, built around the site of St Mary’s Chapel (from the 15th century referred to as St Joseph’s) with a separate monastery upon the Tor. Little is known of its Saxon development but by the time of Dunstan’s Abbacy in the mid tenth century considerable rebuilding and reform had taken place. His new eastern extension of the existing c700 church was probably the burial place of the three Saxon Kings; Edmund, Edgar and Edmund Ironside. The hamlet of Edgarley to the east traditionaly dates from this time. The chapel whose ruins were buried under the 18th century Pennard Road at Chapels, near the end of Ash well Lane, was dedicated to Dunstan. Another early site was at the island of Beckery, to the north west of Northover.
By Norman times the Abbey was the richest in the country. The second of two successive Norman churches was destroyed, with most of the buildings of the Abbey and the town, in the great fire of 1184. The extensive rebuilding continued for the next 350 years, extending in a westerly direction in a newly laid out precinct. The precinct wall was rebuilt cl420 and forms the base of the present walls in Chilkwell Street and Silver Street. The precinct had four gates; one in Silver Street opposite St John’s Church; Chaingate, crossing Magdalene Street near Chaingate House; the Almonry Gate opposite Somerset House and the main gate, now partly restored, next to the Town Hall.
When completed, only a few years before its destruction, the Church was claimed to be the longest in Christendom north of the Alps. It was noted for its extensive library and school and for the help given to the poor. At the dissolution in 1539 the annual income was valued at over £4,000 and 11,000 ounces of gold were sent with the furnishings and valuables to Thomas Cromwell just before the execution of Abbot Whiting on the Tor.