I learn the most interesting things from the random stuff that shows up in my Facebook feed. Tonight I came across the Kowloon Walled City, a real place that existed in Hong Kong until it was demolished in 1993. No need to say much about it here since a simple search will yield enough info to keep you busy for hours, except that I strongly suspect that this was at least part of the inspiration for the hive cities in Warhammer 40k. Just look at the pictures to see what I mean.
Of course, this being GW, they cranked things up to 11, with their version being populated by upwards of billions of inhabitants; the real-world version boasted a much more modest 33,000 at its peak. And there are probably lots and lots of skulls littering that thing too, you just can’t see them from this distance.