Ba Sing Se is the largest city in the Earth Kingdom in the animated series Avatar: The Last Airbender. It is known for its massive walls that protect it from outside threats and its complex social structure, which includes a variety of districts and cultures. The city is divided into upper, middle, and lower tiers, each with distinct characteristics and lifestyles. The city is also famous for its Dai Li, a secret police force that maintains order and suppresses information. Ba Sing Se serves as a central location in the series, representing themes of control, secrecy, and the struggle for freedom.