In the eighth century a writer named Nennius produced a "History of the Britons," written in Latin. This contained many stories about Arthur and his doings, and on it was based the "Chronicles of the British Kings," written by Geoffrey of Gloucester about 1135,
The "Chronicles" were translated from the original Latin into French by Wace in 1150, and back into English by the monk Layamon in 1205.
At each step additions were made as fancy prompted. During the Middle Ages French romances on the Arthurian legends were popular.
In Wales, stories about Arthur formed part of the "Mabinogion," a series of Welsh legends.
The question as to whether King Arthur was or was not an historical personage arises. It is more likely than not that a British chief of that name really existed and fought against the Saxons. But all the stories about the Round Table and the Knights are evidently mere legend, as "knights" of the type of the Arthurian romance did not exist till centuries after the times of British Arthur himself.