I first heard about Kowloon Walled City from a friend who gave me an awesome photo book (City of Darkness) about it. Imagine a 6.5 acre urban jungle, with buildings for days, yet no streets, no cars, only lots of little lanes, footpaths, and alleyways. You could hardly see the sun on most days because this former fort was so jam packed.
The place was something like 70-80% Teochew! Apparently lots of new poor (illegal?) immigrants from Diosua: came to eke out a living in Hong Kong and spent their days in this super dense enclave. Eventually triads, drugs, and prostitution moved in in the 70s and 80s and HK had to demolish it. It must have been a sight to behold - and really dangerous.
Nowadays, it's a big city park - next time I'm in HK, I'd love to visit.
If anyone has any other info to share, I'd love to learn more about the "City of Darkness".