The Kowloon Walled City is famous for having been one of the few structures that defied any sense of reality and survived outside the rule of law. While it was demolished between 1993 and 1994, while it stood it was famous for unlicenced practice of medicine, prostitution and the rule of gangs. Despite its haphazard construction, however, there was some sense of order towards the latter part of its existence, with postal service and police rounds as well as a rudimentary sewage system. In 1987 there were apprently 33,000 inhabitants within its 6.5-acre confines (or about 120 times the density of New York City today).
The amazing detail depicted below owes its existence to Terasawa Hitomi, who illustrated the pages in the book Daizukan Kyuryujyou. While the Japanese book may be hard to get a hold of, "City of Darkness" by Lambot and Girard is a fantastic resource. There are also a variety of documentaries on YouTube, including a German one (which I cannot find in good quality). If you don't mind Japanese, I recommend this one. I assume all responsibility for any faults in translation. Please email me if there are blatant omissions or mistakes, or supplemental information.