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The Black Wall or Black Walls is a great oval of fused black stone built two hundred feet high in the eastern half of Volantis, often called "Old Volantis".
Built by the Valyrian Freehold when Volantis was no more than an outpost of their empire, it is considered the heart of Volantis and only scions of the Old Blood able to trace their ancestry to Valyria are allowed to dwell there.
The Black Wall is so thick that six four-horse chariots can race around its top abreast, which is done annually to celebrate the founding of Volantis.
The Black Wall protects a large labyrinth of:
For two generations Lys, Myr and Volantis were ruled from within the Black Wall by triarchs, but that endeavor ended when the "tiger" faction tried to conquer Tyrosh and were defeated by a coalition.
No outlander, freeman, or foreigner is allowed inside the Black Wall unless they are invited by the Old Blood. Women of good birth who dwell in ancient palaces behind the Black Wall are allowed to vote. Many inside the Black Walls keep the old gods of Valyria, but R'hllor is favored outside the Black Walls, both by slaves and freedmen.
A Dance with Dragons
While writing dragonlore on the Shy Maid, Tyrion Lannister wishes for Galendro's The Fires of the Freehold and hopes that Volantis has a copy in a library. He thinks he may find a copy if he can find a way inside the Black Wall to the city's heart.
While playing cyvasse with Tyrion in Selhorys Qavo Nogarys reveals that Daenerys Targaryen's smashing of the slave trade in Slaver's Bay has deeply affected those that dwell behind the Black Wall and beyond:
Behind the Black Wall, lords of ancient blood sleep poorly, listening as their kitchen slaves sharpen their long knives. Slaves grow our food, clean our streets, teach our young. They guard our walls, row out galleys, fight our battles. And now they look east, they see this young queen shinning from afar, this breaker of chains. The Old Blood cannot suffer that. Poor men hate her too. Even the vilest beggar stands higher than a slave. This dragon queen would rob him of that consolation.
While chained to a wall in a room in the Merchant's House Tyrion sees that one of the two windows looks towards the Long Bridge and the black-walled heart of Old Volantis across the river. The widow of the waterfront tells Tyrion that the Old Blood will have their dogs and children voting before any freedman.