That Camelot is based on Camulodinum is only one suggestion made by some scholars, and not by very many. There are linguistic problems in getting from Camuldodinum to Kaamelot, and “Kaamelot”' or “Kaamalot” and similar forms with a doubled “a”, both pronounced, is the normal early form.
In an case, Camelot is seldom mentioned in Arthurian legend until the late prose French romances, indeed only in one French romance, “Le Chevalier de la Chariot'', and in one German romance, Wolfram von Eschenbach's “Parzival”. Whether Camelot in some sense really existed is a separate question from whether the Knights of the Round Table existed, who are mentioned in many romances that don’t mention Camelot.
The name “Arthur Pendragon” is I believe a Victorian invention. That “Pendragon” is a title and applied to Arthur is not true in the medieval accounts. I believe it first appears in literature in Tennyson’s “Idyls of the King”.
The word “Pendragon” means “head of the dragons”, not “Head dragon”. It may refer to a leader of the dragoons, that is to a cavalry force known as the dragons from their Roman dragon standards, long wind socks with a serpent face on one end. It is a surname of Uther in the medieval texts and never applied to Arthur or to any other character save in Robert de Boron’s “Merlin”. In Robert de Boron’s “Merlin”, Uther’s elder brother is named Pendragon rather than Aurelius Ambrosius. This is probably an error arising from a misunderstanding of Wace’s account where Ambrosius is usually just called “the King” and some lines referring to Ambrosius, and Uther Pendragon have been misunderstood:
“Aureles primes rei sera
E par puisun primes murra
Uther, ses freres, Pendragon
Tendre emprés la regiun.”
That is:
“Aurelius will be king first
And through poison will die first
Uther, his brother, Pendragon
Will afterward hold the region.”
Stories about Arthur meeting at Stonehenge and having 20 followers are not from medieval sources either. They may have been invented by some modern novelist.
The Round Table itself is never mentioned in Welsh texts, save in “Y Seint Graal” which is a translation of two French romances and in Welsh versions of Geoffrey of Monmouth. So it is very doubtful that Welsh tradition knew anything about the Round Table except from French accounts.
Ancient Celts dined sitting or squatting in a circle around a central hearth and did not use tables. Even the Welsh word for table is “bwrd” borrowed from Old English “bord” meaning “board” but used also with the meaning “table” as the medieval table was a board set on trestles. Presumably when the Welsh began adopting the custom of eating off tables from the English, they also accepted the English word.
One can imagine some story teller explaining that Arthur’s warband ate setting in a circle and some of those hearing the account naturally imagined a table, as everyone, so they thought, ate off tables. Thus the Round Table may have been invented. And then the idea of an order of the Knights of the Round Table would have been invented.
The Round Table is first mentioned in surviving texts in Wace’s “Roman de Brut” in 1155.
Of course, if Arthur himself was a real person (which some deny, not without reason), he would have had a war band or a troop of soldiers. So Arthur's knights would be, in a sense historical. Early Welsh accounts refer to his followers Kai and Bedwyr, who appear medieval French accounts as Keu and Beduer and in the 14th century Morte d’Arthur by Sir Thomas Malory as Sir Kay and Sir Bedivere.
But most knights of Arthur listed in Welsh sources don’t correspond to the knights who appear in medieval French sources. A few do correspond, such as Welsh Caradoc Brechbras to French Carados Briebras, and Welsh Owain ap Urien to French Yvain son of Uriens. However the historical Owain flourished at the end of the fifth century or the beginning of the 6th century, long after the death of the historical Arthur. Again, such history as might appear is very garbled. The story of Carados in French sources is very fantastic.
In some cases the Welsh seem to have identified heroes of French legend with heroes from their own tradition, identifying their Gwalchmai ap Gwyar with the French Gauuain, their Gereint ap Erbin with the French Erec li fils del Roi Lac, their Peredur with the French Perceval, and so forth.
In short, while the idea of the Knights of the Round Table probably comes from the followers of the historical Arthur, very few of those historic followers (if any) would actually appear among the Knights of the Round Table in later legend which only emerges to our view around 1170 when Chrétien de Troyes romance “Erec et Enide” was completed.
We don’t know that history might lie behind some of the Knights of the Round Table in medieval romances, because we don’t know enough of the true history of their supposed era. Similarly in most case we don't know what truth lies behind tales of Charlemagne’s knights. The suspicion is that most of the material that comes down to us is invention or borrowings from stories of other personages.