Kowloon walled city was demolished in 1993 after a lengthy eviction process. The settlement was insanely populated due to large scale migration of Chinese during world war 2 and occupation of Hong kong by British.
Most of the settlement was built haphazardly by the settlers themselves. As it was not governed by any authority, crime flourished along with illegal trading. The population was an incredible 3.25 million people per square mile!!