Thinking about the world's only functioning arcology: an apartment building in Whittier, Alaska.
See, in 1964 Whittier was hit by an earthquake and tsunami that destroyed a bunch of housing. But also, coincidentally, it had a large building that had been built when the town had a major navy base, and it had been sitting empty for a few years. So the entire town just moved in. It also houses the town general store, school, church, etc. At this point most of the population is in this one building.
