Penrith is in Cumbria in northern England.
John Leland, in his Itinerary from the mid 1500's, states--"Less than a mile from Penrith, but in Westmoreland, are the ruins of what is supposed to have been a castle. It stands on what is virtually an island, less than a bowshot from the river Lowther on one side and the river Eamont on the other. This ruin is called by some the Round Table, and by others Arthur's Castle"
The earthwork itself dates from 2000-1000 BC. It is only local legend that provides a King Arthur link.
It may have been a prehistoric meeting place. There is a circular ditch with a round plateau in the center, having a diameter of 50 or 60 yards. There were originally two entrances to the central "table" that spanned the ditch, one northwest and the other southeast. The northwest entrance has been destroyed by the modern road.