The Westernlands is one of the nine major regions of Westeros. Prior to Aegon’s Conquest, the Westernlands was known as Kingdom of the Rock, one of the original Seven Kingdoms. At the start of the GoT series, the Westernlands are ruled by House Lannister from the seat of Casterly Rock. The most notable feature of the Westernlands is what lies beneath – gold and silver. Because the Westernlands are so rich in gold and silver, whoever controls them possesses an immense wealth that is unrivaled. Over the years, House Lannister has used its control of the Westernlands and prodigious wealth to buy the power they have attained.
The Westernlands border both the Riverlands and the Reach, and extend west to the Sunset Sea, leaving it vulnerable to invasion from the Iron Islands to the west. The land of the Westernlands is not very fertile nor very populous; its location is not particularly strategic and its terrain is mild compared to other regions of Westeros. If not for the gold and silver beneath its lands, the Westernlands likely would have never become a significant part of Westeros.
Like the rest of Westeros, the Westernlands were originally settled by the First Men, and ruled by the ancient family of House Casterly, after which Casterly Rock derived its name. House Casterly would lose its land during the Andal Invasion. However, unlike other kingdoms which the Andals conquered by war, it is said that Casterly Rock was won over by a trickster named Lann the Clever, using nothing but his wit. After losing its land, House Casterly would go extinct, while Lann the Clever would found House Lannister of Casterly Rock. For 6,000 years, House Lannister would rule as kings of Kingdom of the Rock, until Aegon’s Conquest, during which King Loren Lannister, the last King of the Rock, would bend the knee to Aegon after the Field of Fire. House Lannister would remain overlords of the Westernlands and wardens of the west.
Casterly Rock and the Westernlands would remain one of the most powerful regions of Westeros under the Iron Throne, until Tytos Lannister, father of Tywin Lannister, came to power. Tywin’s father was a weak man and allowed other small-houses of the region to rebel against House Lannister in a quest to control the Westernlands. The rebellion was led by House Reyne of Castamere, a small town of the Westernlands. Though still young, Twin Lannister marched an army to Castamere and crushed the rebellion. He killed every member of House Reyne, eliminating their bloodline forever. This decisive event was put into a song called The Rains of Castamere, which romanticize the military dominance of Tywin Lannister over the now extinct House Reyne. It was this song that was played at the Red Wedding and signaled House Frey to start the slaughter.
Casterly Rock is a great castle from which House Lannister have dominion over the Westernlands. Casterly Rock is one of the largest castles in all of Westeros, and it is set upon a large hill, which once produced vast amounts of gold. It is located along the western coast of Westeros and is accessible by sea. Casterly Rock is named after House Casterly, an ancient house that once ruled over this land thousands of years ago. However, during the Andal Invasion, Lann the Clever tricked House Casterly out of their lands, using nothing but his wits. He went on to found House Lannister from the seat of Casterly Rock.