Popular tradition has evinced her faithfulness in transmitting from age to age the superstitious belief that Fawdon Hill is the royal residence of the "Queen Mab" of Northumberland and all her elfin courtiers, and that the picturesque grounds adjacent are the scenes of the moonlight gambols and midnight revelries.
This is followed by pages of equal wordiness and an excruciating poem, which you may read on Google Books. It's from the 'Metrical Legends of Northumberland' by James Service (1834).
The fort is also connected in various books to the battle of Otterburn - but I think this is more historical speculation than local folklore.
Here's something that more obsessive visitors to the vicinity might want to check out. The first bit comes from a letter from 1729.
..a discovery that has lately been made in the grounds of Otterburne in this county.
There was a large cairn of stones, computed to about 60 ton, which they had occasion to lead off; when the stones were removed, they discovered at the bottom, a large stone, rough and undressed, laid upon the ground, in the form of a grave-stone, with smaller stones wedged in between it and the ground, wherever there were any interstices..
There were ashes and charcoal in the cavity underneath.
"Those who are wishful that all remains illustrative of our early ancestors should be preserved, will be gratified to know that the large stone above mentioned is still either entire or very nearly so. It was conveyed to Otterburne Walk Mill, when the cairn was cleared away; and at present (March 1842) it forms, and has formed for upwards of a century, the landing to a stone stair at the east end of the dwelling-house. It is of a darkish blue or grey colour, seemingly hard, and only a few inches thick... we earnestly hope it may long continue in a state of perfect preservation."
(from 'R White's Manuscripts' and reproduced in 'The local historian's table book' v1 (1843) - p268. It's on Google Books.)
Could it still be there at Otterburn Mill - which is still there and thriving.. their website says the main buildings date from the mid 18th century - which more than allows for Mr White's observation in the 1840s. Ever hopeful - it'd just be nice to think it were still there.