Because Glencairck is based on Celtic mythology, King Arthur is one of the great figures of its history. But there is the Arthur of Mallory and the French romantics, and then there is the Arthur of Welsh myth. Guess which mine is going to be more like.
Some other things that are unique to the Arthur of Glencairck: he is the uniting figure, the one who brings the four kingdoms together, and creates the five fifths, with himself as the Ard Righ, the High King. And then, because Arthur always has an element of the tragic in his story, he almost breaks it all apart again, mostly due to pride. It takes two more weak Ard Righs before one comes along that makes Arthurs vision more permanent: Brian Boru.