It was once considered the densest settlement on the planet - with houses upon houses stacked on top of each other.
If you didn't know the area, you'd find it impossible to find your way around, with buildings so closely packed together and dark corridors snaking around the city.
According to CNN, Kowloon Walled City was once home to around 33,000 people - but its residents were said to call it something a whole lot more sinister: Hak Nam, or the City of Darkness.
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And you can see why - the alleyways around the city were so small you could barely fit through them, while houses were built on top of each other so far up that they could block out the light.
The Hong Kong city has a strange origin story. It was reportedly intended as a military fort - but was sent into a weird limbo when it didn't fully fall under British colonial rule of Hong Kong, meaning residents were pretty much free to do what they wanted there.
No administrative help meant the city fell into disrepair, and with no legal guidelines to follow, houses were built on top of each other - meaning it became hugely overcrowded and densely populated.
Photographer Greg Girard spent time documenting the city before it was destroyed, telling CNN it was "a huge monstrosity of buildings".
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He said: “Quite often houses were built by building onto the next building, punching out walls to use their staircases. A lot of them didn’t have access to air or open space, because they were enclosed in the center of the structure.”
The wild appearance means it looks more like something out of Blade Runner rather than cities we're used to living in today.
But Girard told CNN it wasn't that different inside, saying: “People were doing very ordinary things. It’s just that all these ordinary things were happening in an extraordinary place.”
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The city doesn't exist nowadays - it was torn down in the 90s and replaced with a public park. But if you did want to visit the site it was once on, you'd be able to visit a small museum looking at the history of Kowloon Walled City.
The city might be no more, but it's had a huge cultural impact. A level of Call of Duty: Black Ops is dedicated to the Kowloon Walled City, while a neighborhood called the Narrows in the 2005 film Batman Begins was said to be inspired by the city.