King Arthur was a legendary British ruler reputed to have led a struggle against the Germanic Saxon invaders around the year 500 AD. There are no contemporary or primary sources of evidence for his reign, but this lack of factual information has only added to his significance. He became a British symbol of an idealised ruler at a period of crisis, and in the absence of precise details or historical records about his actual life, fantastic myths were able to embellish his story and take hold.

Although he was mentioned briefly in a few earlier accounts, such as Historia Brittonum (Nennius c.830), King Arthur first became a figure of enduring interest and popularity following Geoffrey of Monmouth's Historia Regum Britanniae (c.1131), which introduced him in a pseudo-historical way as a great and mystical warrior, who defended Britain from human and supernatural enemies. 

The legends about King Arthur, his Court of Camelot and the chivalry of the Knights of the Round Table flourished in the 12th & 13th centuries, the heyday of real knights in armour. Norman knights, particularly the Crusaders, had a serious image problem at the time: They were rightly seen as ruthless, unprincipled killers, selfishly seeking enrichment and loot. The early Arthurian romances combined propaganda with effective public relations, and may well have been intended to improve both the conduct and the reputation of medieval knighthood and militant Christianity.

There was renewed interest in Tudor times, when chivalric jousting was a popular sport but irrelevant to real battle or warfare following the invention of firearms. King Henry VII  named his first son Arthur and sought to associate his own lineage with ancient royal glories of a bygone era.

King Arthur by Howard Pyle from The Story of King Arthur and his Knights, 1903

In Knight for a Day Henry meets King Arthur in the year 533, a mature and wise king nearing the end of his reign. He's ruling over a disintegrating realm, menaced by the evil Sir Mordred's treacherous rebellion and the Anglo-Saxon invasions that were threatening to overwhelm all of England.