Belsay Castle is a 14th century stone three storey tower house, founded by John de Strivelyn. A two storey Tudor mansion was added to the west side and later the mansion was given a Jacobean west wing, now ruinous. The impressive unaltered tower, is crowned by four round corner bartizan's, standing above the machicolated parapet. In the vaulted basement is a kitchen with a well and above is a hall and a solar, both with fireplaces and tall traceried windows with seats. There is a north wing of four storeys and in the southwest corner, a spiral stair and six small rooms.

The castle's three-storey keep was constructed of a rich yellow sandstone by John de Middleton during the reign of Edward III. Attached to the tower is a 17th-century wing with a stone inscription over the porch stating that 'Thomas Middleton and Dorothy his wife builded this house, anno 1614'.