Marlborough Castle, locally known and recorded in historical documents as The Mound,[1] was an 11th-century royal castle located in the civil parish of Marlborough, a market town in the English county of Wiltshire, on the Old Bath Road, the old main road from London to Bath (grid reference SU18376866).

[3][4] In 1067, William the Conqueror assumed control of the Marlborough area and set Roger, Bishop of Salisbury, to building a wooden motte-and-bailey castle, sited on the prehistoric mound.

[7] However, Stephen besieged the castle in July 1139, lifting it the following month before the defenders capitulated so that he could lead a campaign in another part of the country.

He then stayed at the castle in 1149 while in talks with David I, King of Scotland about a potential northern alliance to attack Stephen's forces in Yorkshire.

The earliest record of royal expenditure on the castle dates from 1175–76 when £43 was spent of building materials to create accommodation for the king.

[12] From 1223 to 1224, Eleanor of Brittany, cousin of Henry III and with a better claim to the throne according to primogeniture, was briefly kept there as a state prisoner, during which time the council suggested additional horsemen and crossbowmen.

[15] It was replaced in 1683–84 by the "new house" for his grandson Charles Seymour, 6th Duke of Somerset, apparently to designs by John Deane, a surveyor of Reading, Berkshire.

In the 18th century it was the beloved residence of Isabella, Countess of Hertford, the patroness of William Shenstone and James Thomson.