The ruins of the old castle at Sherborne date from the early 12th century, when the castle was built for Roger de Caen, Bishop of Salisbury. He was a man of great importance who served as Chancellor and subsequently justiciar to King Henry I. However when Henry died there was conflict over the succession, with Henry's daughter Matilda contending for the throne against her cousin, Stephen. Sherborne was seized by Stephen in 1135, and it remained in royal hands until the reign of Edward III, when Bishop Robert Wyville paid to get the castle back into the hands of the church. In 1592, Queen Elizabeth transferred the lease to Sir Walter Raleigh. He had little use for the castle but made use of the estate where he built a new residence, Sherborne Lodge, that now forms part of the new Sherborne Castle beyond the lake to the south.

The old castle saw its final use during the Civil War when it was twice held for the King, in 1642 and 1645. On the second occasion it managed to hold out for 16 days against a siege by the forces of General Fairfax. By the following October its defences had been dismantled to prevent any further use against Parliament.