The link with Arthur is probably early 14th century for that is the time the Cliffords became the owners of nearby Brougham Castle. They claimed descent from the Welsh kings and therefore Arthur. It is significant that after their arrival their castle of Mallerstang became known as Pendragon Castle”.
Archaeological sites of the Lake District
T. Clare
Moorland Publishing Co.
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According to Stukeley, The Round Table was housed in a building:
An unattached circular beehive-type structure, built of forty courses of ashlar blocks...these blocks of hewn stone were set without mortar upon an elevated, square stone platform. The stone platform itself seems to be set upon an earthen mound. The worshiper entered the edifice by means of an unroofed portico of two stone slabs, a smaller upon a larger, the width of the doorway. The door opening was enclosed by a true arch constructed of voussoirs, wedge-shaped blocks pointing toward the center or top of the arch.
What he is describing looks like the San Stefano Rotondo - or could be Stonehenge!