Demolished in 1993, the Kowloon Walled City was a self-contained illegal settlement in Hong Kong. By the early 1980s, Kowloon Walled City had an estimated population of 35,000. It started out as a Chinese military settlement within the then British colony, but evolved into a anarchic enclave, isolated from the rest of the island. Being a lawless land,
the city was notorious for its excess of brothels, casinos, opium dens, cocaine parlours, food courts serving dog meat, and secret factories. The Kowloon Walled City was also infamous for its high number of unsanitary dentist clinics, since this was where unlicensed dentists could operate without prosecution. source
Obviously there are a number of movies that are directly inspired by this imagery, such as Blade Runner and The City of Lost Children.
A whole city was formed by people building on top of each other, with almost no centralized planning or governance:
The streets were illuminated by fluorescent lights, as sunlight was rare except for the rooftops. The only rules of construction were twofold: electricity had to be provided to avoid fire, and the buildings could be no more than about fourteen stories high, because of the nearby airport. A mere eight municipal pipes somehow provided water to the entire structure (although more could have come from wells).
At that time, it had 50,000 inhabitants on 0.026 km², and therefore a very high population density of 1,900,000 / km². It was allegedly the most densely populated spot on Earth.
Compare this density to Paris, which has 24,783 people per square kilometer!
This book is supposedly the definitive source for photographs of the amazing ad-hoc architecture and life that evolved within the city walls.
If you can't afford or find this book (like me) this is an excelent web resource
