HBO's working name Bloodmoon, will take place thousands of years before the events of Game of Thrones, in a time called the Age of Heroes. This is when a number of noble houses were first formed, including House Stark and, yes, House Casterly.
Because of that timeline, it always seemed likely that House Casterly would appear in the Game of Thrones prequel.
The reason House Casterly was never featured in Game of Thrones is because the house had long since died out, but that's a problem the new series doesn't have, and fans will likely become familiar with them when the Game of Thrones prequel finally arrives. Before that happens, here's everything you need to know about House Casterly.
What We Know About House Casterly From The Books
There is almost no mention of House Casterly in George R.R. Martin's A Song of Ice and Fire books, which further explains why they weren't mentioned in Game of Thrones either. That in a way is why Martin and showrunner Jane Goldman are working in this time period, because there's pretty much a blank slate. Even the bits and pieces that have been written or said about the Dawn Age and the Age of Heroes are simply the stuff of legend that's been ed down, meaning much of it might not be true or as people believe.
Still, there is some established history of House Casterly, albeit not from Game of Thrones but rather the companion book The World of Ice and Fire, which was written by Martin alongside two of his frequent collaborators, Elio M. García Jr and Linda Antonsson. In this, Martin writes about the Casterlys in his chapter on The Westerlands.
It's here we learn that House Casterly was formed by a man named Corlos, son of Caster, who lived in a village near what is now known as Lannisport. It's said that Corlos was a huntsman, and when a lion began preying on the sheep in his village, he tracked it down and slew it using only a spear. Legend has it that he killed the lion and his mate but spared the cubs, which pleased the old gods so much that they sent sunlight into the lion's cave, revealing a wealth of gold. Corlos began to mine there, moving inside the cave and fortifying its entrance to defend it from thieves and rivals, becoming the first Lord Casterly in the process. Over the course of years and centuries, the new generations of House Casterly would dig deeper into the cave, carving stairways, halls, and galleries and transforming the giant stone into a holdfast that dwarfed every castle in Westeros. As Martin writes: "Though never kings, the Casterlys became the richest lords in all of Westeros and the greatest power in the Westerlands, and remained so for hundreds of years. By then the Dawn Age had given way to the Age of Heroes."
House Casterly's Sigil
It's unknown what exactly House Casterly's sigil was, but we can make an educated guess. The story of House Casterly coming into wealth and nobility has a lion at its core, which would be the exact kind of thing a Westeros house would put on a sigil. A lion is, of course, the symbol of House Lannister, and since they took over from the Casterlys then there's a great chance they simply kept the lion sigil too.
This fits with one of the Game of Thrones. That means the Hosue Casterly sigil could be a lioness, one of the animals killed by its founder, which is then adapted into a lion by the Lannisters.
The Lannisters Stole Casterly Rock From The Casterlys
Although House Casterly isn't mentioned in Game of Thrones, we do know one key fact about them that directly relates to the main series: they were the occupants of Casterly Rock before House Lannister. As mentioned, it was the Casterlys who built Casterly Rock, but during the Age of Heroes it was taken from them by a person called Lann the Clever, who is described in The World of Ice and Fire as a "golden-haired rogue... from out of the east."
This is one of the only direct mentions we have of House Casterly in the main A Song of Ice and Fire books. In the chapter Eddard VI from A Game of Thrones, we find Ned Stark reading a book on the lineages of the Great Houses of Westeros, which includes this mention of Lann and House Casterly: "In the songs, Lann was the fellow who winkled the Casterlys out of Casterly Rock with no weapon but his wits, and stole gold from the sun to brighten his curly hair." It's not much, but it's the basis of a legend that has many possible explanations, perhaps to be explored in the Game of Thrones prequel.
The most common tale is that Lann found a secret way inside Casterly Rock, one so small he had to take off his clothes and coat himself in butter to squeeze through. Once inside, however, he was able to whisper in the ears of sleeping Casterlys, stealing from one and planting it on another, howling like a demon, setting up traps and more to pit the family against each other and convince them the Rock was haunted. Another version states he filled the entrance with vermin to drive out the Casterlys; another still that he somehow smuggled in a pride of lions, who simply ate the Casterlys.
Finally, there's the notion that Lann worked in some capacity for the Casterlys, impregnated his daughter, married her, and then when Lord Casterly died without a son, the Rock and all its wealth ed to the daughter and thus Lann. The final version is the one The World of Ice and Fire lends the most credence to, citing Archmaester Perestan's belief of such, but whichever is true it's a fact that House Casterly suddenly disappeared, and House Lannister arose in its place at Casterly Rock, where it ruled from then until the time of Game of Thrones.
Is Naomi Watts' Character A Member Of House Casterly?
The set photos from the Game of Thrones prequel haven't just revealed a potential new sigil, but also given a first glimpse at Watts' role in the Game of Thrones prequel is currently unknown, but it has been revealed she's playing "a charismatic socialite hiding a dark secret."
Based on that description, the resplendent gold costume she's wearing, and Watts' own appearance, then it leaves a couple of distinct possibilities: she is either playing a member of House Casterly or House Lannister. However, since Martin has said the Lannisters aren't there yet when the Game of Thrones prequel begins, while Watts is in the first episode (this filming is just for the pilot), then it makes her being a Casterly more likely, and that the location in Italy is being used for Casterly Rock.
Still, so much is unknown about House Casterly that it leaves a number of options open for the specifics of Watts' character, but she has to be someone very important. It may be that she is the Casterly daughter who marries Lann the Clever, thus helping found House Lannister, while one popular Game of Thrones prequel theory is that she's really playing Lann, which would be a major twist on the old legend.
House Casterly's Role In The Game Of Thrones Prequel
It seems certain that the Game of Thrones prequel is introducing House Casterly, but just how will they fit into Bloodmoon, The Long Night, or whatever name the series ends up going by? It's quite possible that, at first, we'll simply be seeing the truth of their origins play out. That means Corlos fighting the lions, discovering the gold, and building Casterly Rock as House Casterly's home. Within that, they can put some twists on the old legends, and still maintain the politically-motivated, inter-House squabbling fans of Game of Thrones are used to seeing.
Still, it's likely that House Lannister will emerge at some point in the Game of Thrones prequel we're going to be given the real story of how Lann winkled Casterly Rock from the Casterlys, and how one of Game of Thrones biggest and most important houses came into existence.