Cornwall by Sabine Baring-Gould — Read

Before the Norman Conquest there were no masonry built castles in Cornwall, only stockades of wood surmounting earthworks or piled up masses of stone uncemented. The usual Saxon _Burh_ was a mound, surmounted by a structure of timber, reached by a bridge or ladder from a base-court that was encompassed by moat and mound and stockade. The Norman system of building a castle was to erect a round or square keep, a massive structure of stone, on the mound that had formerly been surmounted by a wooden structure, and to surround the base-court with a stone wall. Within this were erected the necessary domestic buildings. Very generally the entrance to the court was strongly defended by a second tower. The style of castle was greatly altered in the reign of Edward I but of such Edwardian castles there are no examples in Cornwall, save the poor fragment of Tintagel. The Normans built a castle at Launceston, and there the circular keep standing on a lofty tump of rock, artificially shaped, is of their construction, but the ruined buildings below, with the gateways and walls of the base-court, are later.

The castle of Trematon also consists of a "motte" surmounted by a circular keep, and a base-court with square tower at the entrance, with an archway.