Ba Sing SeThis info taken from The Avatar Wiki.Ba Sing Se
is the massive capital of the Earth Kingdom which encompasses nearly all of the nation's northeastern corner. The city is by far the largest in the World of Avatar, essentially being more of a small country than a mere city. Ba Sing Se means "Impenetrable City"; it is so named for its world-famous walls, the gates of which have no hinges or other hardware, and are opened by the use of earthbending. Within the Outer Wall lies a large expanse of farmland, Lake Laogai, and several mountains. The Inner Wall encloses the city itself. The metropolis is also notable for having a unique system of walls dividing the populace into different parts of the city based on social status.History
Ba Sing Se is arguably one of the oldest settlements in the world. A five-thousand year old map to Ba Sing Se was found in Wan Shi Tong's spirit library, which suggests the city is at least that old. Early in its history, Ba Sing Se was a small subterranean city carved out of rocks by earthbenders. As the benders excavated further, they came across beautiful illuminated crystals and began to mine them. The crystals currently serve as a common light source for Earth Kingdom inhabitants. Eventually, as the metropolis' expansion reached the surface, the catacombs were abandoned. The city continued to grow and eventually became the capital of the newly-unified Earth Kingdom. Although Ba Sing Se is the largest city in the world, no figures have been given for its population.
Ba Sing Se has been hailed as the greatest fortress in any of the four nations, having remained unconquered for centuries prior to the Hundred Year War. During Chin the Conqueror's largely successful campaign to unify the Earth Kingdom under his control, Ba Sing Se was the only territory to remain unconquered besides the peninsula where Avatar Kyoshi lived. Apparently, Chin would rather face the Avatar herself than try his luck at the Great Wall of Ba Sing Se.
After Chin's death, a peasant uprising erupted against the corrupt and inefficient government of the 46th Earth King. The peasants felt that the Earth King's role was outdated and that he did not represent their interests. Avatar Kyoshi managed to quell the revolt by forcing the King to accept the peasants' demands in exchange for creating the Dai Li to protect the government's interests.
The city's Outer Wall had only been breached on three occasions: The first was by Fire Nation military forces led by General Iroh, the legendary "Dragon of the West". This attack, a part of his 600-day siege of the city, was quickly driven back by the city's guardian forces after he was disheartened by the death of his son, Lu Ten. Years later, the Fire Nation used a great drill to bore through the Outer Wall, but Aang and his friends stopped the machine before the army could take advantage of the hole. By the end of the spring of 100 ASC, however, Ba Sing Se had fallen to the hands of the Fire Nation through an internal coup d'