To be quite frank I do not know anything about this earthwork (and there are no details yet on Magic). It comes with its own megalithic folklore though, which is well brought to life in the following extract:
As [Saint] Samson and his party were about to descend from Laneast Down, they observed a bald hill on the left, now Tregeare.. The hilltop was thronged with people engaged in an idolatrous revel. Samson recalled what Winiau [his cousin] had said to him, that the natives were still immersed in devil worship, and he at once descended from his wagon, and taking with him two of his monks.. made for Tregeare, and in his zeal, ran up the hill.
He found the people dancing round an upright stone, and the chieftain of the district was looking on with approval. Samson remonstrated. The people good-humouredly explained that no harm was meant; they were merry-making as was their immemorial custon; but some advised Samson to mind his own business. Certain of the company were angry at his interference.
Samson persisted.. at this moment a boy of noble birth who was mounted on an unbroken colt, and was careering about the hill, was thrown, fell on his head, and lay stunned on the sod. This drew off the attention of the revellers. Samson went to the lad, made people stand back, and prayed for the child's recovery. Happily, the boy opened his eyes and stood up.
The people, supposing that the Saint had raised him to life [like heck they did] became more willing to listen to him. Instead of destroying the menhir, Samson cut a cross upon it. The revellers gave up their dancing for that year, to resume it on the next anniversary [my italics].
The stone is no longer on Tregeare height, but a very rude granite cross stands by the wayside from Laneast Down to Tregeare.
From p 156 of Baring-Gould's 'Lives of the British Saints' (1913). The text is a translation of an older document. Samson was supposed to have lived c500AD.