Kowloon Walled City was an ungoverned settlement in New Kowloon, Hong Kong. The city was famous for its criminal activities that even the police couldn't do anything about it. The city was controlled by Triads and had high rates of prostitution, gambling, and drug use. The city was demolished in April 1994. Despite of that, its famous still persists in literature, video games, and movies.
While the city no longer exists, you can experience a little bit of the city, in Japan, in a place called Digital Kowloon Walled City.