Ba Sing Se is the monolithic capital of the Earth Kingdom, encompassing nearly the entire northeast corner of the country. It was the last great Earth Kingdom stronghold after the fall of Omashu. Ba Sing Se means "impenetrable city", an honorary reference to its two insurmountable walls, the gates of which have no hinges and can therefore not be opened in any way aside from the use of earthbending.

At one point, however, Princess Azula of the Fire Nation did manage to break through the wall using a drill and infiltrated the city.