Kowloon Walled City was an ungoverned, densely populated de jure Chinese enclave within the boundaries of Kowloon City, Hong Kong. Originally a Chinese military fort, the walled city became an enclave after the New Territories were leased to the United Kingdom by China in 1898. Its population increased dramatically following the Japanese occupation of Hong Kong during World War II. By 1990, the walled city contained 50,000 residents within its 2.6-hectare (6.4-acre) borders. From the 1950s to the 1970s, it was controlled by local triads and had high rates of prostitution, gambling, and drug abuse.
There is an interesting documentary about it called:
Kowloon – The Walled City (1988)
6.9/10 – imdb rating
It’s a crazy place, so dense even sunlight couldn’t reach lower levels. Down there was night 24/7.
In January 1987, the Hong Kong government announced plans to demolish the walled city. After an arduous eviction process, and the transfer of de jure sovereignty the enclave from China to Britain.
De jure sovereignty means that a government has a legal right to control over a particular territory. De facto sovereignty, however, refers to whether that government actually has control over the territory.
The demolition began in March 1993 and was completed in April 1994. Kowloon Walled City Park opened in December 1995 and occupies the area of the former walled city. Some historical artifacts from the walled city, including its yamen building and remnants of its southern gate, have been preserved there.