It is so called to distinguish it from the ‘new’ castle, a great mansion first built by Sir Walter Raleigh but much enlarged since. Roger de Caen, Bishop of Salisbury, the most magnificent prelate of his age erected the old castle. He lost his influence and possessions for supporting the Empress Matilda against King Stephen, and despite the protests of subsequent bishops, the castle stayed in royal hands for the next two centuries.
In 1592, it was leased to Sir Walter Raleigh, who started to modernize the castle before opting to erect its successor nearby. The abandoned castle was reoccupied on behalf of the King during the Civil War. It was stormed by Sir Thomas Fairfax after a two-week siege and slighted to prevent any further military use.
Like some other Episcopal palace-fortresses of the Norman period, Sherborne consists of a residential quadrangle surrounded by a defensive outer bailey. The outer bailey covers a large octagonal area, or rather a rectangular area with canted corners, bounded by a deep ditch and curtain. There were five square flanking towers, all but one surviving to some extent. Mural towers were an advanced feature for Bishop Roger’s time but there are not enough of them to flank the long curtain comprehensively.
The best preserved is the gate tower at the west-south-west angle, which seems to have been the original main entrance into the castle. A square keep occupies one corner of the inner quadrangle, though not much above the vaulted ground floor still stands. There are remains of three sides of the quadrangle, especially the north range which contained an ornate chapel over a vaulted undercroft, but the hall opposite was probably pulled down by Sir Walter Raleigh to achieve the fashionable E-plan. To the west are foundations of a second quadrangle added after the castle returned to the bishop of Salisbury.