Sherborne Old Castle was built in the early twelfth century by Roger, bishop of Salisbury, who served Henry I as a leading royal administrator. The fortress stood above the Dorset town and abbey, combining a strong enclosure, towers, halls, chapel, and service buildings. Roger's wealth and influence enabled construction on a scale that alarmed King Stephen during the civil war of the 1130s. Stephen seized Sherborne and Roger's other castles in 1139, bringing the site permanently into royal control.

Medieval kings and officials used the castle intermittently, but maintenance declined as political and military priorities changed. The Crown leased the property to tenants, and rooms were adapted rather than continuously renewed as a royal residence. In 1592 Elizabeth I granted Sherborne to Sir Walter Raleigh. He attempted to modernize the old castle but found its defensive plan inconvenient. Instead, he converted a nearby hunting lodge into a new house, beginning the building now called Sherborne New Castle.

After Raleigh's fall and execution, the Digby family acquired the estate in the early seventeenth century and expanded the new house. The contrast between the two castles became sharper: one remained a medieval Crown fortress, the other an elite residence arranged for comfort, display, gardens, and estate management. During the English Civil War, the Digbys supported the king. Royalist troops garrisoned the Old Castle, whose walls again offered strategic value despite decades of reduced maintenance.

Parliamentarian forces besieged Sherborne in 1645 during the final campaign against Royalist strongholds in the southwest. Artillery breached the defenses and forced surrender. Parliament ordered the Old Castle slighted, leaving the roofless towers and walls seen today. The New Castle survived and remained the Digby family home. Later landscaping, including work associated with Capability Brown, incorporated the medieval ruin into views from the newer residence, transforming military destruction into an aesthetic feature of the estate.

The Old Castle is now conserved in public guardianship, while the New Castle remains privately owned and occupied. Together they show the movement from fortified episcopal and royal power to the country-house culture of landed families. Archaeology and documentary evidence recover the vanished rooms of the old enclosure, and the new house preserves later interiors, collections, and landscape design. Sherborne is unusually valuable because two neighboring buildings make architectural change visible: Raleigh's rejection of the medieval fortress and the Civil War's destruction prevented the older site from being absorbed into the later residence.