Stable City was a settlement in Canterlot, built around Stable 1.
Stable 1 was breached by Pink Cloud over the first century of its operation. This exposed the inhabitants to Pink Cloud and Balefire radiation. Many died, some survived and escaped Canterlot whilst others became ghouls. The ghouls made a settlement for themselves inside and outside Stable 1, establishing Stable city. They trade with anypony foolhardy enough to brave the Pink Cloud.
In the last ten years, the ghouls of Stable City have been at war against the alicorns who are looking for the Black Book. Star Sparkle, the mother of Twilight Sparkle, lives at the edge of the settlement, shunned by the majority of its residents for Twilight's actions during the war. Stable City guards employ the usage of broadcasters as defense.