Cities today are mostly rooms in (home/office/store) units in buildings separated by roads. Besides roads, cities provide city services like power, water, sewage, and telecom. Buildings channel these services to units and add more like lights, air/heat, security, and elevators. Units are managed by particular orgs, who may add further local services like receptionists and IT, while people and small groups have their own rooms, over which they have some discretion.
Humans are more productive and engaged when we can interact with more others faster and more easily. The main way we achieve that today is via many adjacent tall buildings with supporting mass transit. But few buildings are very tall, and those roads take up much of the area, while remaining a bottleneck of movement. So for a long time another possibility has captured many imaginations: megastructures, such as this from Blade Runner:
Here more units could be spatially closer, and reached faster via many-level transit. An extreme version of this ideal is elaborated in my book The Age of Em. And this ideal was partly achieved in Hong Kong’s Kowloon walled city slum, destroyed in 1994: