Over the past six seasons of Game of Thrones, we’ve traveled extensively through the lands of George R. R. Martin’s imagination. We’ve seen the towers of Winterfell and the permafrost north of the Wall; the castles of Qarth and the water gardens of Dorne; the heights of the Vale and the ruins of Valyria.
First, let's rewind. By the end of season 6, Daenerys has finally mobilized an army to cross the Narrow Sea. Dany, her three dragons, and her army of Unsullied and Dothraki troops are on their way to conquer Westeros. But where will they land? What stronghold will they conquer first? Daenerys may be the mother of dragons, but she doesn’t know the first thing about Westeros — other than the fact that she wants to rule it.
"Back in season 2 when Stannis was headed for King's Landing, Tyrion told Lord Varys that back when he was a young adult he was put in charge of all of the drains and sewers of Casterly Rock by [his father] Tywin," wrote Professor-Reddit, the author of the theory.
"Now back in the season 7 trailer, we saw Unsullied troops sacking a castle belonging to House Lannister, leading to the theory that it could be Casterly Rock itself (the home of House Lannister). Could all this experience that Tyrion had manning the drains and sewers lead him to ordering his men to enter the castle through a secret route — a concept we have seen so many times in so many episodes (think of Mereen, Yunkai, King's Landing back when the defenders had to get rid of that ramming device)."
From what we know of Casterly Rock, it’s just about as strong as Cersei Lannister's demeanor. The castle sits atop a natural rock formation said to be three times taller than the Wall. In more familiar measurements, Casterly Rock is taller than the Empire State Building, and 7 miles wide.
Given these impressive dimensions, the Lannister stronghold is virtually impenetrable.
“[Casterly Rock] supposed to be impossible to take through force. Even Visenya [Targaryen] is supposed to have said that it'd have been hard to conquer even with dragons. Having the Unsullied simply storming the gates and taking it over would be a bad way to do it,” Reddit user genkaus wrote.
But sneaking through the castle’s massive sewers and tunnels like a sneaky virus? That’s a far more promising possibility — not to mention that the sacking of Casterly Rock would be yet another sweet victory for Tyrion in his quest to undermine his sinister sister, Cersei.
Unfortunately, we can’t comb through A Song of Ice and Fire for details that will fuel this theory. Though the Lannister siblings remember their home in flashbacks, the stronghold is never actually visited in the present moment in the books. According to Martin, that might change, too.
“I haven’t brought the action to Casterly Rock yet. You’ve never seen Casterly Rock. I think both of those locations will play key roles in the books to come,” Martin said in 2012.
If season 7 really is the season of Casterly Rock, then the Lannister twins — and their massive fortune garnered from the gold mines beneath the rock — are in big trouble.
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