1. The walls.

A city is defined by its walls. They mark the divisions between interior and exterior spaces, separate the small and large volumes of air that create a sense of "inside" and "outside."

Walls are punctuated by openings. We see the large ones and think of them as specific functions (doors and windows). But in truth walls are like dense meshes, or nets, full of little holes that the air (and things upon the air) can move through. Walls are permeable in ways we don't intend them to be.

Rare is the wall that functions in such a way that it completely excludes. That would need to be a very high, very thick wall indeed.