But if the well-fortified Renaissance city could be seen, for the sake of argument, as something like the Hummer of military urbanism, what is the city-as-Bruce-Lee? A city that is lean, even physically underwhelming, but brilliantly fast and highly flexible? What is the city that needs no defensive walls at all?

There are a variety of possible answers here, all of which would be interesting to discuss; but I’m most struck by the possibility that the phenomenon recently dubbed the feral city is, in a sense, the anti-fortress in exactly this spatial sense.

In a 2003 paper for the Naval War College Review, author Richard J. Norton describes the feral city as “a great metropolis covering hundreds of square miles. Once a vital component in a national economy, this sprawling urban environment is now a vast collection of blighted buildings, an immense petri dish of both ancient and new diseases, a territory where the rule of law has long been replaced by near anarchy in which the only security available is that which is attained through brute power.” From the perspective of a war planner or soldier, Norton explains, the feral city is spatially impenetrable; it is a maze resistant to aerial mapping and far too dangerous to explore on foot. Indeed, its “buildings, other structures, and subterranean spaces would offer nearly perfect protection from overhead sensors, whether satellites or unmanned aerial vehicles,” Norton writes, creating, in the process, an environment where soldiers are as likely to die from rabies, tetanus, and wild dog attacks as they are from armed combat.

I’m led to wonder here what a twenty-first-century defensive literature of the feral city might look like—from temporary barricades to cartographically incoherent slums experimenting with limited forms of micro-sovereignty. If the feral city is a city with no external walls but an infinite interior—endless spaces made of oblique architecture and indirect streets—then its ability to defend itself comes precisely through letting invaders in and disorienting them, not by keeping people out.

So if a city does away with defensive walls altogether, what specific spatial strategies are left for it to protect itself? For instance, can a city deliberately be made feral as an act of preemptive self-defense—and, if so, what architectural steps would be necessary to achieve such a thing? Channeling Archigram—or perhaps even Cisco—we might call this the insurgent instant city complete with its own infrastructural practices, its own rogue designers, and its own anti-architects.

How, then, could the spatial practice of urban feralization be codified, and what architectural lessons might be learned if this were to happen?

Michael J. Lewis, describing the treatises on display at the CCA nearly two decades ago for The Geometry of Defence, refers to “fortification literature” or “the literature of the fortification,” including the publishing practices peculiar to this—for its time—top secret field of study. Privately circulated manuscripts, incomplete essayistic reflections, and even word of mouth only gradually solidified into full-length narratives; only at that point were they intended to communicate finely tuned, often firsthand, military knowledge of the city under siege to anyone who might want to discover it, whether that was a king, a layperson, or an enemy general (indeed, much of the literature of fortification went on to the form the core of an emergent field known as urban planning).

In another fifty, one hundred, or even five hundred years, then, will there be a defensive literature of the feral city, its systematic description, techniques for its defence (or obliteration), and its urban logic (or lack thereof)? Even if only on the level of urban form, this would be a fascinating journey, going from Castriotto’s and Maggi’s indirect streets to whole cities gone wild in the name of resisting outside intervention.