Casterly Rock, nicknamed the Rock, is a castle and the seat of House Lannister. The capital of the Westerlands, it overlooks the harbor of Lannisport and the Sunset Sea. Nearby strongholds include Kayce and Feastfires to the west, Sarsfield to the north, and Cornfield and Clegane's Keep to the south.
Casterly Rock is carved out of a great stone hill colossal rock beside the Sunset Sea. It is popularly believed to resemble a lion in repose at sunset.
The Casterlys of antiquity built a ringfort on the peak, and as millenia have passed its natural defenses have been expanded with walls, gates, and watchtowers. The base of the Rock contains large sea-carved caverns. The stone has been mined for thousands of years, so there are hundreds of mineshafts in the depths of the Rock, as well as yet untouched gold veins. The Rock itself has been measured at three times the height of the Wall or the Hightower of Oldtown.
The Rock is almost two leagues long from west to east, and contains tunnels, dungeons, storerooms, barracks, halls, stables, stairways, courtyards, balconies, and gardens. Its port has docks, wharves, and shipyards and is accessible by longships and cogs. From below the Rock thunder can be heard, where the sea comes in. In the bowels of the Rock are rooms where caged lions were once kept, and cells for the worst prisoners.
- The Golden Gallery contains treasures of the Lannisters, including gilded ornaments and walls.
- The Hall of Heroes is where the Lannisters and their close kin who have died valiantly are interred. The armor of Lannisters of old are also displayed in the hall.
- The Lion's Mouth, the main entry to Casterly Rock, is an enormous natural cavern reaching two hundred feet high. Its steps are now wide enough for twenty riders.
- The Stone Garden is a godswood with a twisted weirwood a tenth the size of Raventree Hall's.
The region was probably inhabited by children of the forest and giants before the arrival of the First Men, and numerous animals have made it their home as well.
Legend says that Casterly Rock is named for the family that ruled it in the Age of Heroes, the Casterlys, who no longer exist. The Lannisters trace their rule over it back to Lann the Clever, who tricked the Casterlys out of their castle and claimed it for his own. Some claim that Lann still haunts the fortress, which has never fallen.
The Kings of the Rock from House Lannister took their title from their seat, and members of the family founded nearby Lannisport. During the Andal invasion, the Lannisters eventually invited Andals into their realm and brought Andal youths to serve at Casterly Rock. According to a lost scroll, Valyrian sorcerers believed their downfall would come from the Rock's gold.