Everyone who climbs Steep Hill in Lincoln has seen Lincoln Castle. It stands where William the Conqueror planted it in 1068, high on the old Roman upper city, its twin mottes and curtain wall commanding the whole plain. It is one of the best-preserved castles in England, and it still does the Crown's work — the Crown Court sits there to this day.

Almost nobody knows that Lincoln once had a second castle.

Down by the river, at the vulnerable (from a defensive point of view) south-eastern corner of the lower city, there stood for perhaps two or three generations a fortress called Thorngate Castle — castellum de Tornegat. It was not royal like Lincoln Castle (which at one time housed the sheriff, the constable/ castellan, and the bishop). Thorngate belonged to a family, held in their own right, and it existed in a state of quiet tension with the great royal castle on the hill. And then, in the space of a decade, it was pledged away as a bargaining chip in a civil war and demolished so completely that today we cannot even say for certain where it stood.

This is the story of that lost castle: of the family who held it, the war that consumed it, the forced marriages and shifting loyalties tangled around it, and the arguments that still divide historians about where it was and who really controlled it. It is, in miniature, the story of the whole terrible period we call ‘the Anarchy’ — and, as so often, it is a story with a woman at its centre whom history has almost entirely forgotten.