Directly between the Williton and Carhampton locations of the Life of St. Carannog is Daw's or Dane's or Dart's Castle (http://www.pastscape.org.uk/hob.aspx?hob_id=188490). Excavations here show the fort to be of Saxon origin (http://sanhs.org/Documents/130/03McAvoy.pdf). The name Daw(e) is supposedly from a landowner in the 16th century. Dane's Castle is self-explanatory - and, in fact, the archaeologists believe this fort was built to defend the coastline from Viking marauders.
The fort is built right on the cliff edge and is actually missing a large portion that has fallen or eroded away onto the shore below. Fairly extensive mud and sand extend below it and to its east and west. It thus qualifies particularly well as a Din, fort, plus OCo. plur. *traitou, 'beaches, strands' (see Charles Thomas, Note 24, p. 325, And Shall These Mute Stones Speak?) or 'Dindraithou.'
Dart could be another family name, perhaps based on the name of the River Dart of Dartmoor fame in Devon. However, it could also represent a simple metathesis of trait, 'beach, shore': Trait > Dra(i)t > Dart. I'm now checking with all the relevant authorities in Somerset regarding the Dart name and if any new information is forthcoming from those sources, I will add it to this blog post.
When on reads the story of Carannog's floating altar, which Arthur wishes to use as a table, one is struck by the fact that it washes up first at Carhampton and then at the Guellit stream (Williton). Arthur's interaction with the saint and this altar demands that Dindraithou be close to these places AND on the shore of the sea, flanked by sands associated with the mouths of rivers. The only fort that fits this description is Dart's Castle.
We need not insist on finding an actual sub-Roman or early medieval site for Arthur in this instance. Obviously, we are dealing with a story found in a saint's life. Hagiography can hardly be considered historical and, indeed, we must expect more than a little poetic license. It is quite possible the author of St. Carannog's Life knew of this fort near the Carannog sites and simply decided for dramatic effect to insert Arthur into the fiction. He may have been ignorant of the fact that the fort went back no further than the Anglo-Saxon period.
And interesting point about the altar/table: "whatever was placed upon it was thrown to a distance." This suggests a confusion on the part of the author. Dolmens (Breton dolmen means 'table stone')/cromlechs are, in folklore, often referred to as quoits. This is because it was believed that giants (or great heroes) had flung the rocks that made up the funeral monuments, including their capstones, during such a game. So the notion that objects were "thrown" from the table is a reference to the table itself - or the table's legs (= orthostats) - being thrown, thus creating a quoit.
There appear to have been such monuments near Williton. The following is from An Exploration of Exmoor and the Hill Country of West Somerset by John Lloyd Warden Page (1890):
"A mile inland, close to Wiliton, is a field, or rather several fields, known as Battlegore, traditionally, as its name implies, the scene of a battle. In them are the remains of three large mounds, though one is now ploughed nearly level with the field, and another has been reduced by one-half by a hedgerow. The largest is close to the road.
From time immemorial the tale has been handed down that here the Danes fought with the Wessex men. A tradition, also unfortunately dating from time immemorial, states that much armour and many weapons have been discovered in these fields. But who found them, and what became of them, is as unknown as their period and fashion. The only weapon taken from the spot that I have seen is a remarkably fine bronze celt which would go some way to show that it was a British rather than a Danish battleground.
Collinson refers to 'several cells composed of flat stones, and containing relics,' as having been found in these tumuli, to which he gives the name of Grab-barrows. From this it would appear that they were chambered tumuli. I venture to think, however, that he is mistaken, except perhaps with regard to the mound now nearly levelled, inasmuch as neither of the existing barrows have been properly explored.
Close to the barrow near the road are two enormous stones, the one lying on its side, the other leaning against the hedge, as well as a third and smaller block, nearly concealed by brambles. As there are no similar blocks in the vicinity, they must have been brought here for some definite purpose, perhaps to mark the grave of some notable chieftain. Or, perchance, they are, as certain antiquaries opine, the supports of a British cromlech. The local story is that they were cast there from the Quantocks by the devil and a giant, who had engaged in a throwing match. The print of Satan's hand still marks the leaning stone.
This stone was upright some forty or fifty years since. It was toppled against the hedge by some young men anxious to test the truth of the legend that it was immovable."
[For additional information on this barrow cemetery, see the Pastscape entry at http://www.pastscape.org.uk/hob.aspx?hob_id=188542]
Diagram and text from British Fortifications Through the Reign of Richard III: An Illustrated History by Jean-Denis G.G. Lepage (McFarland, Dec 1, 2011)