Horn Hill is the seat of House Tarly in the Reach. According to legend, the castle was built on Horn Hill by the brothers Harlon the Hunter and Herndon of the Horn, twin sons of Garth Greenhand. In the Age of the Hundred Kingdoms, before the Reach was united under the Kings of the Reach, the kingdom of the western marches extended from Horn Hill to Nightsong.

Located on an eponymous hill, the castle lies south of Highgarden and a hundred leagues northeast of Oldtown among the thickly-wooded foothills of the Red Mountains.