Generally the fort is thought to originally date from the early Iron Age, with the single bank and ditch being doubled in the last couple of centuries BCE.
Huddlestone’s Table (information from “Cleeve Hill: The History of the Common and its People” – David H. Aldred 1990 [Alan Sutton Publishing Limited]):
Traditionally the stone is said to mark the spot where King Kenulf of Mercia took leave of various important guests after the 811 dedication of Winchcombe Abbey. In 1779 an article about the stone appeared in “Gentlemen’s Magazine” linking it in true antiquarian style with Druids and so on.