From Reddit comment about the Chinese city of Chongqing:
The og city of Chongqing was built along the Yangtze River hemmed in by mountain plates. In the 20th century, the city-long a local hub between Qinghai/Sichuan/Southern China-sprawled like wildfire, from the riverside it crept up to the mountain plates, with mountain residents finding it onerous to haul ass down to the city center and vice versa. By the 1980s-2000s, high rises began to be built, some of which met level with the mountain districts. The mountain citizens couldnt be assed to travel all the way downriver to get to these buildings so what did they do? Connect the upper floors & rooftops with a series of skyways, walkways, and eventually metro-rails. In the process the nearby mountains got hollowed out for tunnels to serve skyways and metrorails.
End result: Chongqing becomes a multi-layered city. Where instead of just thinking in 4 directions, you also have to worry about which “floor” you are at in the city.