What was the shape of King Arthur's table?

Actually, contrary to popular belief, King Arthur never had a round table, in fact he had never owned a table in his life or had anyone else he knew because tables had not been invented. They ate squatting or sitting on the floor around a central hearth. The Welsh did not even have a Word for table until they borrowed the Word "bord" (meaning "board") from Old English. A table is just a board on trestles. The Welsh adapted the Word as "bwrd".

Presumably some teller or Arthurian tales was explaining that Arthur's men ate sitting in a circle, and some of his hearers, unable to conceive of noble knights eating sitting or squatting on the floor of a castle, invented the Round Table in their own minds, and probably thought that was what the story teller was describing.