Tintagel’s true scale wasn’t understood until the 1980s. After centuries of denial, a drought led to a grass fire so intense it burned away the apparently thin turf covering over 200 ruined buildings. No one had noticed them before. :^)
If indeed there was a King Arthur, it’s more likely that every distorted piece of folklore referring to various post-Roman local lords (Romano-British at best, probably) wound up grafted into a single quite recent (15th c) legend.
Vortigern was historical, and probably wasn’t rival or archenemy of Arthur, but formed a good bit of the basis for the King Arthur composite. There’s actually a very old medieval burial monument in England (a part that used to be Wales) that states that the historic king buried there is a descendant of Vortigern.
Another part of the composite seems to be Magnus Maximus, a 4th c Roman governor of Britain who also vied for the emperor’s job, went to the mainland (as one story has Arthur doing), and died in battle trying.
Even in what is now Scotland there’s an Arthur’s Seat, which probably has nothing to do with the rest of the legend and was given the name much later on.
https://en.wikipedia.org/wiki/De_Excidio_et_Conquestu_Britanniae
https://en.wikipedia.org/wiki/Category:Arthurian_literature_in_Latin
https://en.wikipedia.org/wiki/Nennius
https://en.wikipedia.org/wiki/Matter_of_Britain
https://en.wikipedia.org/wiki/Vortigern#Pillar_of_Eliseg
https://en.wikipedia.org/wiki/Geoffrey_of_Monmouth
https://en.wikipedia.org/wiki/Chr%C3%A9tien_de_Troyes
https://en.wikipedia.org/wiki/Welsh_Triads
https://en.wikipedia.org/wiki/Mabinogion
Watch this guys video of King Arthur.
There WAS no King Arthur. Anything close would have either been Irish or Roman, but, there wasn’t one man.