Rugged gold country behind Casterly Rock, rich enough that House Lannister's wealth has become a figure of speech in half the Seven Kingdoms — and merciless enough, two rebellious houses learned, that the rains still weep for it in a song sung at nearly every wedding since.
The westermen trace their nobility to Lann the Clever, who by every telling talked, tricked, or otherwise relieved the Casterlys of their own Rock without a drop of blood spilled — the sort of founding legend a house built on gold mines is only too pleased to leave unverified. Whatever its truth, Casterly Rock and the mines beneath it have made House Lannister the wealthiest family in Westeros for longer than most other great houses can trace their own lines.
Two centuries and more after that founding, when a soft-hearted Lord Tytos let his bannermen mistake his generosity for weakness, Reyne of Castamere and Tarbeck of Tarbeck Hall rose against their liege; young Tywin Lannister's answer was thorough enough that the ruins of Castamere are sung about to this day, and no westerland house has repeated the experiment since.
In the timeline
SourcesTWOIAF · The WesterlandsASOS · Tyrion
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Where is The Westerlands?
Rugged gold country behind Casterly Rock, rich enough that House Lannister's wealth has become a figure of speech in half the Seven Kingdoms — and merciless enough, two rebellious houses learned, that the rains still weep for it in a song sung at nearly every wedding since.
Is The Westerlands from the books or the show?
Book canon. This entry follows George R. R. Martin's novels and histories, and notes where the television series diverges rather than following it.