Foss Castle is a late 11th century earthwork motte and bailey fortress, founded by Nigel Fossard. Standing against the deep gorge of Mickleby Beck, the large circular flat-topped motte is encased by a wide ditch and crowned by the remains of a rampart. Protected on the south and west by steep slopes, the motte is flanked by a northern triangular bailey and a larger southern rectangular bailey. A north-eastern ditch and the steep scarp of a western stream also gave defence to the open side of the castle. Abandoned in the early 13th century, when Robert de Turnham founded Old Mulgrave Castle, Foss has a dense cover of trees, so it is best viewed in winter.