Imagine a city with 30 times the population density of Manhattan. Imagine it was a lawless den of vice in which drugs, prostitutes and illegal gambling were all available. Imagine it sat directly under the landing path of one of the world's busiest airports.
That's Kowloon's walled city, as profiled by a number of websites listed over at oobject.com
...with no streets and little daylight, it was a rat infested, cockroach ridden filthy labyrinth. KWC was a no-man’s land that fell neither under British Jurisdiction nor Chinese
It had 161 unregulated doctors and dentists along with food producers from whole pig roasters to the suppliers of most of Hong Kong’s fish balls. Most of the people that lived in KWC never left.