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.