Kowloon Walled City

Kowloon Walled City documentary - https://www.youtube.com/watch?v=Lby9P3ms11w

Like the 'appearance in creative works' section points out, it's a popular inspiration for stories and games with a bit of an anarchistic streak. Gibson's Bridge trilogy and Shadowrun: Hong Kong used it in their settings, and you can see why. It's a natural fit for Cyberpunk; heck, it was more or less a manifestation of the genre.

But ... it didn't actually work all that well, did it? It was a huge slum, which plenty of cities have. It was just unique in its small urban footprint.

You can see something similar in Caracas, Venezuela, where people live in unfinished skyscrapers. There's no lifts nor official electricity and there are grocery shops every nth floor with prices growing as everything is delivered on foot.

Also known as the city of darkness: http://cityofdarkness.co.uk/

William Gibson visited the city and described it in his 1997 work, Idoru:

"There was a place near an airport, Kowloon, when Hong Kong wasn't China, but there had been a mistake, a long time ago, and that place, very small, many people, it still belonged to China. So there was no law there. An outlaw place. And more and more people crowded in; they built it up, higher. No rules, just building, just people living. Police wouldn't go there. Drugs and whores and gambling. But people living, too. Factories, restaurants. A city. No laws."

Mildly-related: HK[0] is an in-dev videogame about a cat in a not-Kowloon-but-similar walled city.

0: https://hk-devblog.com/2015/10/07/first-step/