Cities can be viewed as information architecture systems. Here, ärchitecture" is used in the sense of computer architecture -- it refers not to the design of buildings, but to how the components of a complex system interact. Information exchange includes the movement of people and goods, personal contact and interactions, telecommunications, as well as visual input from the environment. Information networks provide a basis for understanding living cities and for diagnosing urban problems. This paper argues that a city works less like an electronic computer, and more like the human brain. As a functionally complex system, it heuristically defines its own functionality by changing connections so as to optimize how components interact. An effective city will be one with a system architecture that can respond to changing conditions. This analysis shifts the focus of understanding cities from their physical structure to the flow of information.