Kowloon Walled City was a densely populated, largely ungoverned settlement in Kowloon Hong Kong. Originally a Chinese military fort, the Walled City became an enclave after the New Territories were leased to Britain in 1898. Its population increased dramatically following the Japanese occupation of Hong Kong during World War II. From the 1950s to the 1970s, it was controlled by Triads and had high rates of prostitution, gambling, and drug use. In 1987, the Walled City contained 33,000 residents within its 6.5-acre (0.03 km²; 0.01 sq mi) borders.

The Walled City of Kowloon has no visible wall around it, but it is as clearly defined as if there were one made of hard, high steel. It is instantly sensed by the congested open market that runs along the street in front of the row of dark run-down flats—shacks haphazardly perched on top of one another giving the impression that at any moment the entire blighted complex will collapse under its own weight, leaving nothing but rubble where elevated rubble had stood.

—Robert Ludlum, The Bourne Supremacy, p. 149[45]

I think it’s one of the most curious example of self organized society on such a tiny plot of land though a dark and gloomy one.