Aberffraw Castle was the seat of Rhodri Mawr, one of the great kings of Wales, in the early Middle Ages. Nothing of it remains–it seems to have shared a similar fate with Aber Garth Celyn upon the death of Llywelyn ap Gruffydd. We do have information that some of it lasted until 1316 when the last remaining timbers were stripped to repair Beaumaris (or Caerfarnon) Castle, both part of Edward’s ring of iron castles that he built after the conquest of Wales.
My favorite Castles of Wales site doesn’t even have Aberffraw in its database because, quite literally, nothing of it remains. The book by Paul Davies, ‘Castles of the Welsh Princes’, states only: ” . . . a modern village sits on top of Aberffraw; the occasional discovery of richly-carved stones hints at the vanished splendour of the great court.” (p. 19)
‘Aber’ means ‘estuary’ in Welsh, thus the large number of place-names that begin with it. Aberffraw (which thus means, ‘estuary of the Ffraw River’) Castle was built in the 6th century by Maelgwyn, the King of Gwynedd. http://www.britainexpress.com/countryside/coast/aberffraw.htm
In archaeolog-ese: http://homepage.mac.com/philipdavis/Welshsites/966.html
“The probable site of a medieval llys, a princely court partly dismantled in 1317. Excavation, 1973-4 (White 1979) & 1979, with further work 1987-8 (White & Longley 1995), recorded part of a rectilinear enclosure with at least one rounded corner. Its ditch had been recut on at least one occasion. It was traced for 70m north-north-east to south-south-west. Intially thought to represent a Roman military work, refurnished in the early medieval period. The apparently curving corner has suggested the presence of a castle mound (see NPRN 400058). This overall interpretation is supported by a radio-carbon determination of about 27-387AD, however scarsity of Roman material, limited to a single scrap of samian and some uncertain sherds, counts against the presence of a fort. It is possible that this was the site of a later Prehistoric style settlement occupied in the Roman period. The site of the llys is otherwise uncertain (see Johnstone 1997, 63; Longley 1997, 45). Two possibly thirteenth century sculptured heads are known from the village (White 1978). Excavations at the traditional site of the llys, about 650m to the west-south-west recorded only C18 remains (see NPRN 15012/401125). (Coflein–John Wiles 12.07.07)”
The Royal House of Aberffraw is something else entirely, though it derives from the establishment of the royal court there. Llywelyn Fawr spoke of himself as from the Royal House of Aberffraw as a justification for his rule of Wales in the 13th century. He could trace his line back to Rhodri Mawr, as the founder of that house in the 10th century. http://house-of-aberffraw.co.tv/