The city of Ba Sing Se has stood untouchable for centuries. It is the largest city on the planet, the hub of Earth Kingdom trading, military, everything. Only has Azula been able to take the city, albeit inside out by generating Dai Lee support.
Today, the city houses many royals and many lower class workers as well. There's the Outer Wall which is the first wall you encounter on your way to Ba Sing Se. This wall went untouched for centuries. Behind the Outer Wall lies a large expanse of farmland, Lake Laogai, and several mountains. The Inner Wall encloses the city itself. The city is also notable for having a unique system of dividing the populace into different parts of the city based on social status