Nevermind the high crime rate. I’m not really concerned about population density per se. Super tall buildings, crammed in next to each other, with narrow alleys between. In some parts street lights have to be on 24 hours a day because sunlight can’t get through. Less necessary to the question, but still of interest, is the extensive network of paths, stairways, and bridges, making it possible to walk significant distances without setting foot on the actual ground.
The city is long gone, but I find it fascinating looking at photographs of it. What’s the nearest equivalent today, if any? It doesn’t have to a whole city, it can just be a section.