The 'fortifications' surely refer to this site? Who knows. The author for all his long-windedness seems to know the lay of the land.
The Ghost of the Black-Dog.
A man having to walk from Princetown to Plymouth took the road which crosses Roborough Down. He started at four o'clock from the Duchy Hotel, and as he walked at a good swinging pace, hoped to cover the sixteen miles in about three hours and a half. It was a lovely evening in December, cold and frosty, and the stars and a bright moon giving enough light to enable him to see the roadway distinctly zigzagged across the moor. Not a friendly pony or a quiet Neddy crossed his path as he strode merrily onward whistling as he went.
After a while the desolation of the scene seemed to strike him, and he felt terribly alone among the boulders and huge masses of gorse which hemmed him in. On, on he pressed, till he came to a village where a wayside inn tempted him to rest awhile and have just one nip of something "short" to keep his spirits up.
Passing the reservoir beds, he came out on an open piece of road, with a pine copse on his right. Just then he fancied he heard the pit-pat of feet gaining upon him. Thinking it was a pedestrian bound for Plymouth, he turned to accost his fellow traveller, but there was no one visible, nor were any footfalls then audible. Immediately on resuming his walk, pit-pat, pit-pat, fell the echoes of feet again. And suddenly there appeared close to his right side an enormous dog, neither mastiff or bloodhound, but what seemed to him to be a Newfoundland of immense size. Dogs were always fond of him, and he of them, so he took no heed of this (to him) lovely canine specimen.
Presently he spoke to him. "Well, doggie, what a beauty you are: how far are you going?" at the same time lifting his hand to pat him. Great was the man's astonishment to find no resisting substance, though the form was certainly there, for his hand passed right through the seeming body of the animal. "Hulloh! what's this?" said the bewildered traveller. As he spoke the great glassy eyes gazed at him; then the beast yawned, and from his throat issued a stream of sulphurous breath. Well, thought the man, I am in for it now! I'll trudge on as fast as legs can carry me, without letting this queer customer think I am actually afraid of him.
With heart beating madly and feet actually flying over the stony way, he hurried down the hill, the dog never for a moment leaving him, or slackening his speed. They soon reached a crossway, not far from the fortifications. When, suddenly the man was startled by a loud report, followed by a blinding flash, as of lightning, which struck him senseless to the ground. At daybreak, he was found by the driver of the mail-cart, lying in the ditch at the roadside in an unconscious state.
Tradition says, that a foul murder was many years ago committed at this spot, and the victim's dog is doomed to traverse this road and kill every man he encounters, until the perpetrator of the deed has perished by his instrumentality.
There are similar legends of the doings of the Black Dog throughout the county, and many wayside public houses have "The Black Dog" for a sign.
From Nummits and Crummits by Sarah Hewett (1900). It's rather dramatised up, I'm sure most Black Dogs aren't so mean. It also reminds me of something I'm rather interested in at the moment, the 21st century tale of the Big Cat (which is also often black).
Description of the site, on Roborough Down next to the Plymouth - Yelverton road (A386):
Early Iron Age Camp.
The camp is formed of circular earthworks; there is no evidence of any stone being used. The outer bank is 100 yards in diameter, the smaller one, or Keep, 100 feet across. To the south the inner bank is still further protected by a sickle-shaped breastwork or agger. The camp is approached by two embanked ways, one from the main road, the other going north east from the camp, crossing both the Buckland road and the main Plymouth-Tavistock road, and can be traced to the south-eastern end of the golf links. The inner bank has been heavily planted with hawthorns and may well have been used as a pound for stray animals. There is also a small secondary earthwork, 50 feet in diameter, across the road opposite the banked trench, and a long embankment. The camp was almost certainly used as a beacon; on Spry's 16th century map of Plymouth Leat it is marked as Rowben Beacon.
The air photograph shows Saunders' map to be conventionalised. The inner enclosure, "The Keep", is not concentric with the outer rampart but lies to the east of its centre : within it appears to be two small circular enclosures with a rectangular enclosure against their north west sides. The "breastwork" appears to be earlier than the outer rampart and to have extended, formerly, to the west where there is a trace of a low bank. The "embanked trench" does not appear to extend beyond the road at SX 50676421.