Following the departure of the Roman army in the late fourth century AD, Anglesey formed the power base of the kings of Gwynedd. Aberffraw on the West coast of Anglesey was the location of the principle court of llys of that dynasty in the early medieval period. In the thirteenth century Llywelyn ap Iorwerth one of Wales's most powerful rulers confirmed it's continued importance by his use of the title 'Prince of Aberffraw'. In 1282 when Edward I defeated Llywelyn ap Gruffydd, the last ruler of Gwynedd many of the buildings of the llys were dismantled and abandoned. There is therefore little evidence of Aberffraw's former significance. However, in the village church there is a finely decorated chancel arch dating from the twelfth century, a clue as to the importance of this little village.
Just south of Aberffraw at Llangadwaladr; housed in the village church is an early memorial stone that commemorates Cadfan who was king of Gwynedd in the seventh century.
The remains of another of the royal courts of the thirteenth century have recently been unearthed by archaeologists near the village of Newborough. Rhosyr (the name of this area in the medieval period) was one of six royal courts on Anglesey. A massive storm in the early fourteenth century partly covered the site with sand; this resulted in the quite exceptional survival of some sections of the complex. This site is unique in that here in Rhosyr it is possible to see the remains of two buildings, part of the perimeter wall and the hall of the llys where the public business of the court would have been conducted.