There’s a treehouse hotel you can stay at in Ohio. It’s a pretty cool idea. You could pretend to be an Ewok. I wonder what the maximum density would be for a sustainable treehouse-based civilization. You could use gondolas to get around, minimizing the transportation footprint. You would still need electricity and water infrastructure, hospitals, etc. You would need food, but maybe you could rely heavily on tree nuts and fruit to provide a lot of it. And obviously, you couldn’t cut down too many trees to build the treehouses or you wouldn’t have enough trees left to build them in.