From A Wiki of Ice and Fire
|House Casterly of Casterly Rock|
|Title||Lord of Casterly Rock|
|Cadet Branch||House Lannister|
|Founded||Age of Heroes|
Legend says the first Lord Casterly was a huntsman, Corlos, son of Caster, who lived in a village near where Lannisport eventually sprang up. This legend tells ups that Corlos slew a lion that had been troubling the village, but he spared its cubs. The gods took favor on him and showed him where to find a huge vein of gold.
The truth of that tale is lost to the mists of time but we cannot doubt that some Casterly found gold inside the Rock and began to mine there. To defend this treasure, the entrances were fortified and over years and centuries Casterlys delved deeper and deeper, following the gold, whilst carving elaborate halls and galleries and stairways and tunnels untill this gigantic stone was transformed into a mighty fastness that dwarfed every castle in Westeros. Though never kings, the Casterlys became the richest lords in all of Westeros and the greatest power in the Westerlands and remained so for many centuries.
When the Dawn Age had given way to the Age of Heroes a golden-haired rogue appeared from the East. This so-called Lann the Clever winkled the Casterlys out of their Rock and took it for his own. The precise manner in which this happened remains a matter of conjecture, as many different stories are told. All that is known for certain is that sometime during the Age of Heroes, the Casterlys vanish from the chronicles and the Lannisters appear in their place.
References and Notes