Fast Castle is a late 12th century earthwork motte, possibly founded by the de Rule family. Standing on the west bank of Rule Water, the mound is partly natural with only the top being artificial. It was probably approach from a northern ford, here its slope is reduced and access to the top was defended by a cross-ditch, which is now only a terrace. The motte is truncated by a road to the south and river erosion to the east and the summit is crowned by the remains of an oval rampart, with a small annexe on the south-west angle. Nearby is Bedrule Castle and 4 miles north-east is Jedburgh Castle.