Arthur was a 6th century Celtic personality who fought against invading Saxons. He is first mentioned in a 6th century poem, the Y Gododdin which was originally written in Briish Celtic in south Scotland. The Gododdin were a tribe with their capital in what is now Edinburgh.
Nennius was a Welsh chronicler of the early 9th century. He refers to King Arthur and calls him warlord. Appointed by Celtic kings. The Annales Cambriae – compiled circa 955 AD – mentions great victory of Badon and the death at Camlann. Geoffrey of Monmouth (c. 100-1155) wrote in Latin Historia Regum Britanniae. It began the development of Arthur as a mythical being. Thence Arthur headed off into European literature via Norman poets – Wace, Chretien de Troyes, and Layamon).
After the historical defeat of Arthur by Anglo-Saxons [see Beresford-Ellis: Celt and Saxon: the struggle for Britain AD 410-937], the BritishCcelts gathered storytellers to tell the sagas of Arthur. Thus – Sir Gawain and the Green Knight was borrowed from the Cuchulain Saga. This story with the same motif is the Feast of Briciu where Cuchulain plays role later given to Gawain.
Arthur and his ‘knights’ appear in one of the Irish myths – e.g., Cuchulain steals the hound of Fionn mac Cumhall – with at least 25 Arthurian tales identified in Irish of the period. In Welsh mythology Culhwch and Olwen is the furst known fully-fledged Arthurian tale. It dates from well before it was first written down during the 11th century.
Three later Arthurian tales – in the Mabinogi – are (1) The Lady of the Fountain; (2) Peredur, son of Efrawg, and (3) Geraint, son of Erbin. Also an appearance in the 10th century poem The Spoils of Annawn – a prototype of the Grail Legend.
Tristan and Iseult [Tristam and Isolde] in Beroul (translated from a Breton source) is probably derived from a Cornish source. The saga of Tristan and Iseult is one of the world’s greatest love stories. The central motif is the traditional elopement tale. In Irish it is known as althedha. King Mark was real. Tristan was real. Mark’s Castle Dore was 2 miles north of Fowey (an earthwork fortification), inhabited from 2nd century BC to 6th century AD.
An engraved stone, at Par and from 6th century BC, and inscribed in Latin states “Drustaus hic iacit Cunnomori filius”. Here lies Drustanus, son of Cunomorus. Drustanus equates with Tristan. King Mark’s full name in the records is Marcus Cunomorus (not derived from the Roman). Mark is Celtic for a horse. In Cornish he is Margh and in Welsh he is March. Cunomorus means ‘hound of the sea’. The first complete Celtic language version (Welsh) of Tristan and Iseult only survives from the 16th century.