interstitial space (the spaces between other things) is fascinating for many reasons, not least the clear way it illustrates our biases & the limits of perspective.
What's more interesting is how hard we find it to transfer what we learn
Take this picture

It's fairly obvious that it's impossible to decide which are the interstitial spaces here - the white stripes or the black - unless we have further information, or perspective.

Yet when we look at maps or schematic representations of, for example, cities we instantly think we know where the interstitial spaces are. They're the brownfield sites, the wasteland, the verges, the dead-end alleys, the weeded concrete between carriageways

But that quickness to label is only there because of perspective. A map is just a map. Take off the labels and hand it to a topologist and see what answers you get about interstitial space.

And this is what fascinates me about our cities (other things as well, but cities are what do it for me), and the assumptions we make about them. Which parts are the essential infrastructure & which are the spaces between is a wholly relative matter

Imagining otherwise leads to the loss of more than richness and understanding - it is can lead us to make statements of value about the importance of those people, creatures, things, ideas that occupy the different spaces & the relative importance of preserving & nurturing them

A very simple (but limited in its own way) way to see this is to look at corridors of steps, rails, ramps, roofs & underpasses between buildings. To the free runner, these are the central structures of a city, the buildings the interstices

The outlines on a map that divide it up - the liminal spaces - are interesting in their own way (and also far fuzzier & more porous than we imagine), but for me they are less interesting because we know that we approach them with extra vigilance & inquiry.

If you find any of this interesting, I thoroughly recommend you take a look at the work of @scotinoz on urban creativity & especially on how criminality is used to define perspective https://unimelb.academia.edu/AlisonYoung