Barnweill Castle is a medieval sub-rectangular earthwork platform, which commands the edge of an escarpment. Giving defense to the once flat-topped motte, are three substantial rock-cut ditches and the natural north-western steep slope. No building foundations can be trace or a causeway across any of the ditches but there are signs of some disturbance to the interior of the platform. 2 miles north-west is Helenton Mote and 3 miles south-east is Tarbolton Motte.