Abergwyngregyn Castle is an earthwork motte, which incorporates some rough stonework. Interestingly this large conical flat-topped mound is constructed from river boulders and is known as Pen-y-Mwd, Y-Mwd in early Welsh means vault or chamber and it is thought to be a 5th or 6th century ceremonial site, which was reused in the 11th century. 5 miles is west is Penrhyn Castle.