The town takes its name from the castle built by Bernard de Balliot and extended by his son of the same name. Between them they erected a powerful stone castle in the second half of the twelfth century, strongly situated on a rock above the River Tees.
Today the castle is an extensive but very ruinous pile. It possesses an exceptional four baileys, all walled in stone during the period of the two Bernards. From the town of Norman arch – once part of a gatehouse – leads into the northern outer bailey, known as the Town Ward. Much of its curtain still stands as well as the vaulted undercroft of the Brackenbury Tower. The southern outer bailey doubles the size of the castle but its defenses are now fragmentary.
West of the Town Ward are the ditch and curtain of the inner bailey, with two flanking towers added by the Beauchamps. To reach the inner bailey it is necessary to pass through a middle ward, then turn sharp right over a deep ditch hewn out of solid rock. This succession of defenses is quite advanced for the twelfth century. Once inside the inner bailey the dominant is the Balliol Tower or keep which projects from the curtain.
This cylindrical tower of ashlar is actually an early addition to the castle, though it could still be the second Bernard’s work as he survived until 1199.
As keeps go, it is a bit of a fraud, because it was not isolated from the rest of the castle. It was entered directly from the vanished solar at first floor level, and the triangular spur projecting from the keep is not a defensive feature but merely a wedge between the two. All the same, the keep is the only part of the castle to survive more or less complete and an unusual domed vault covers its ground floor.
On the Duke of Gloucester’s estate at Barnwell can be seen three successive manorial centers in close proximity. First there are the earthworks of a Norman motte And bailey, now hidden in a clump of trees. Then comes the massive stone ruin of Barnwell Vastle, built by Berengar le Moine about 1265-66. It seems that Berengar took advantage of Henry III’s preoccupation with his barons to build a strong adulterine castle. Berengar later sold his new castle to Ramsey Abbey. It is said he was compelled to do so by Edward I as a punishment for building it without a license. Barnwell remained with the abbey until the Dissolution, when Sir Edward Montague purchased it. He erected the present house, Barnwell Manor, nearby.
The castle is an interesting example of thirteenth century military architecture with some delightfully experimental touches. On a smaller scale, it anticipates the great castles that Edward I would build in Wales in the following decades, and though it pre-dates Edward’s coronation by several years, it is a rare English example of a pure Edwardian castle.
An unusually thick curtain, well preserved except for the loss of its parapet and a single breach on the west, surrounds an oblong courtyard. Circular towers project boldly at three angles, the fourth being occupied by a gatehouse. The two northern towers are quite eccentric as they both have a smaller round tower projecting from them, resulting in a figure-of-eight plan. The prime function of these subsidiary towers was domestic rather than military. They contained latrines serving the apartments in the main body of the towers.
The southwest tower has no projections, but its upper floors are square internally for greater domestic convenience. The latrine for this tower was accommodated in a more conventional manner within the thickness of the curtain.