Omashu

Located... somewhere in the Earth Kingdom continent hidden amid a mountain range is the City of Omashu. Built atop a large platform in the middle of a canyon, the only way to enter the city is by means of flying or the singular stone trail leading up to it. Omashu is a beautiful and very large city filled with civilians, Earthbending guards, a mail delivery system, and more. A wise (but slightly crazy) Earthbending king named Bumi leads them. Because the city is so large, they've developed an intricate system of delivering supplies throughout the city by way of chutes and slides with the aid of earthbending. It's quite the spectacle.