I’ve just learnt about Kowloon Walled City. It’s a really interesting part of Hong Kong’s history. It was a part of the island disputed between the British and the Chinese, but really ruled by organized crime most of the time. It became one of the places with the highest population density in the world (almost 2 million per sq km). And it was demolished in the early nineties. You should check the wikipedia entry to know more.