Castamere was the seat of the ancient House Reyne. It was named after a nearby pool of water.
Castamere began as a mine, similar to Casterly Rock, and this caused the Reynes to be almost as wealthy as House Lannister during the Age of Heroes. The Reynes built curtain walls and two stout towers to guard the entrance. The gold and silver eventually dried up, and the caverns were widened and turned into subterranean corridors and halls. The surface fortifications of Castamere resemble those of a minor lord or landed knight, but nine tenths of the castle is underground.
Kevan Lannister, the second son of Tytos Lannister, served at Castamere as a page and squire to Lord Robert Reyne. Tensions rose between Houses Reyne and Lannister after the War of the Ninepenny Kings, with an exchange of hostages occuring at Castamere and overseen by Ser Reynard Reyne. During the ensuing Reyne-Tarbeck Rebellion Castamere was the site of the last stand of House Reyne. They retreated to the subterranean levels of the castle, but Tywin had them flooded, and the line of House Reyne was extinguished.