Thirlwall Castle is an early 14th century stone fortified L-plan hall-house, founded by John Thirlwall. Standing to roof height, the large rectangular block is of four storey, with turrets and a five storey tower projecting from the angles. Supported on a man-made platform and built entirely of Roman masonry from Hadrian's Wall, there are the remains of a walled barmkin, to the south-west. A mile south is Blenkinsopp Castle and 3 miles west is Birdoswald Bastle.