Castell Aberlleiniog is a late 11th century earthwork motte and bailey fortress, founded by Hugh d'Avranches, earl of Chester. Standing above the Afon-y-Brenhin valley, the steep sided sub-circular mound with its southern bailey, was attacked by Gruffydd ap Cynan in 1094. Nothing further is known of its history until the 1640s when Thomas Cheadle, constable of Beaumaris, turned the castle into a Civil War fort. The stone structure that crowns the flat-topped motte maybe partly Civil War or from the conversion of the site into an early 18th century folly. The square enclosure with its north-west entrance, is flanked on the angles by small round towers, the walls have evenly spaced buttresses and parapets. The earthworks have also been modified but the motte is still encased by a wide ditch and the two small southern mounds are probably the remains of a wide bailey rampart. 2 miles south is Beaumaris Castle.