"at a place called Mur Castell in the uplands of Ardudwy"
Tomen y Mur
Creative Commons Attribution Share AlikeTomen y Mur - Credit: Jeff Buck

Mur Castell (‘Castle Wall’), now known as Tomen y Mur (‘The Mound of the Wall’), is situated near Trawsfynydd in Gwynedd. Originally the site of a Roman fort, it later became the location of a medieval castle, possibly built by William Rufus in the eleventh century. The castle was of the typical motte-and-bailey variety, the motte (or mound) being built within the enclosure of the original Roman walls.