Defence activities frequently have an impact upon the form and function of cities that long outlives the original military necessity that created them. The lines of long, obsolete fortifications, the fields of fire or inundation determined by superseded technologies and the buildings erected to accommodate long-departed armies and their supplies, can remain as visible features of the urban landscape, often shaping the internal morphological structure of the city, centuries after the defence functions which brought them into being have disappeared. Cities whose locations were selected on defence criteria, whose growth was caused and shaped by defence considerations, or whose economies, societies and urban self-image were formed in military service, generally continue in existence once that impetus has disappeared, but with a distinctive legacy, welcome or not, which is directly attributable to the previous defence functions.
Defence is not unique among urban functions in being in a constant state of evolution, and thus making changing demands upon the city. Most urban activities are continuously and relatively rapidly altering their functional requirements to be met by an urban form that can accommodate such changes only slowly and with difficulty. The argument of this chapter is that this inertia, although common to all urban activities—to a greater or lesser degree—is particularly marked with the defence function, which therefore results in past defence activities having an especially influential and longstanding impact upon the city.
The demands that defence makes on cities are characterized by their extreme volatility. The need for defence is generally either pressing and immediate (with the threat to security taking precedence over most other requirements) or is a minor consideration of the long term (which can be sacrificed for other, more immediate, needs). In practice, history has oscillated with great rapidity between these two conditions, so that in time of war or recognized immediate threat of it, enormous demands could be made of cities, their populations and industries, in the name of common