Llys Rhosyr was discovered in 1992 by archaeologist and Menter Môn Heritage Officer Neil Johnstone. It was one of the Royal courts of Llywelyn Fawr, Prince of Gwynedd in the 13th century and is situated in the South West of the island.
It is believed the site was largely abandoned in the years after the Edwardian conquest of 1282 and buried beneath sand dunes in 1330. Since being discovered over a quarter of the walled site has been exposed including the large main hall and an adjacent building thought to have housed the Prince's private rooms. The excavation has also yielded a number of finds, notably pottery, coins and other small artifacts.
Llys Rhosyr is a significant site not least because it is the only Royal Court of the Princes of Gwynedd of which the ground plan remains virtually intact. Those elsewhere were despoiled during the conquest or destroyed in the last century.
In order to protect the site whilst allowing public access Menter Môn supported the consolidation of the old court and people can now walk around the ruins. A car park close to the site was created in Newborough and interpretation boards were erected near the ruins. Further information can also be accessed at the Pritchard Jones Institute also located in the village.