Casterly Rock is a fortress, overlooking the harbor of Lannisport and the Sunset Sea. It is the seat of House Lannister.
The fortress is carved out of a great stone hill and its natural defenses are further enhanced with walls and other structures, In fact, the Rock has never fallen. Legends say 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 the Lann the Clever who tricked the Casterlys out of their castle and claimed it for his own.
Three key rooms within the Rock are the Golden Gallery, the Lion's Mouth and the Hall of Heroes, where the Lannisters and their close kin who have died valiant might be interred. From below the Rock thunder can be heard, where the sea comes in. There is a place called the Stone Garden within the castle.