This is actually an Iron age 'Defended Settlement' - an unusual and distinctively northern type of prehistoric fort. It's defended by the cliffs of Spindlestone Heughs on the south east. The settlement is rectangular and has two annexes, one on the north and one on the west.
The site features in the legend of the Laidly Worm:
An isolated pillar called the Bridle Rock stands out from the edge of the cliff on which tradition says that the Childe threw the bridle of his horse when he went to meet the Worm.
… it is more commonly called the "Spindle Stone" from its shape. .. Near it was the cave of the Worm, but the outer wall has unfortunately been quarried away, and all that remains is a blackened slab of stone forming part, it is said, of the inner wall. A few hundred yards in another direction is the trough which held the milk of the "seven kine" and from which the Worm drank. It is a hollowed stone, some six feet long, two wide and two deep, and now forms a drinking place for cattle.
From various sources, collected in County Folk-lore v4, Northumberland (1904).