Kowloon Walled City, Hong Kong was demolished in 1993. Prior to demolition Kowloon had a population of approximately 50,000 people living in a 6.5 acre area. The City had a population density of 1,900,000 / km2 which is unreal when you compare it to the population density of London at 4,669 / km2 or Hong Kong of 6,206 / km2. Even though Kowloon was located in the British controlled Hong Kong territory China remained control of the city. Because of China’s lack of interest in Kowloon, the city became a lawless land, and was notorious for its excess of brothels, casinos, opium dens, cocaine parlors, food courts serving dog meat, and secret factories. Today the area that was once Kowloon walled city is now Kowloon Walled City Park with a few relics of the walled city preserved a various locations in the park. These relics of the Walled City include 2 cannons, 5 stone inscriptions and 3 old wells. For more information on the history of the walled city and a list of some of the movies that were filmed there check out this Wikipedia
link.