Fun fact: the police weren't allowed to set foot in there during majority of the time that this place existed.

Boring historical background from an Hong Konger:

Hong Kong was ceded to the UK during the Qing dynasty in the mid to late 1800's, the original Kowloon Walled City was excluded from the treaty and remained a Chinese territory (which they continued to use as an outpost. in a way, kind of like an embassy in the modern times). British/HK troops/policeman/government officials were not allowed to enter. Fast forward to the Chinese civil war around the 1930/40s, the Nationalist government lost the war and retreated to Taiwan while the Communist claimed the rest of China. The Kowloon Walled City remained status quo and off limit to the Brits while neither the Nationalists or Communists did anything to it. It didn't take long before all the gangs and underground criminals took notice of the absence of a government there and figured it would be a good place to do their business. Eventually, it turned into the spot to do all things illegal, anything from underground casinos to unlicensed dentists to where you could find restaurants where you could eat dogs.

Throughout its existence, I believe the police only went in there a handful of times, most notably in the mid 70s when they sent a massive show of force in there in an attempt to get rid of the gang activities (which they failed)

This is how it used to look like when it used to really have a wall and before the Japanese tore it down to expand the nearby KaiTak airport during WW2