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Hayford is the seat of House Hayford in the Crownlands. According to semi-canon sources, Hayford is located half a day's ride north of King's Landing along the kingsroad. Hayford sits upon a hilltop and has a stream running alongside the castle. The hidden village of the Lady of the Leaves is located near the Hayford road.
A Feast for Crows
Ser Jaime Lannister camps his forces beneath the walls of Hayford and is asked to supper by the castellan of the castle. Jaime brings several lords and knights with him. Under the moonlight he spars with Ser Ilyn Payne in a small courtyard next to Hayford's armory.