Thousands of years ago during the height of the Ghiscari Empire, Meereen was second only to Old Ghis in wealth and glory. A paradise by the sea if one didn't mind the constant clinking of chains and cracking whips.
Slaves built Meereen, and on their backs, the city rested like a litter.
Then came the dragons of Valyria. The Masters of Old Ghis would not bow, so they burned. The Valyrians tore down their walls, burned their pyramids to ash and sewed their fields with salt and skulls. An old slave woman once told me that the Valyrians intended to break the chains of the Ghiscari slaves, absorbing them into the Valyrian Freehold where every man had a vote.
But the Great Masters of Meereen received their new overlords in the Great Temple, plying them with gold, wine and all the wealth that slavery had bought them, and the Valyrians instead took up the whip themselves.
I do not believe it. As their empire expanded, the Valyrians needed more and more bodies to feed their mines and colonies, where no wage could ever tempt a free man.
The Great Masters merely accommodated their new customers, staying rich as the Ghiscari Empire crumbled. After the Doom fell on Valyria, the Great Masters worried for their fortunes. By this time the Valyrians were their greatest providers and purchasers of slaves. Then the Dothraki horselords swept out of the plains of Essos and proved to be as fond of slaving as the dragonlords before them. The Great Masters grew richer still.
For the first time in its history, the Great Masters alone ruled Meereen and chaos ruled the rest of the world. The fighting pits swallowed men who in previous centuries would have filled the ranks of the Valyrian armies.