A city can be conceived as a complex self-adaptive system. The multiple interactions among its structural elements and dynamic agents, its organization on multiple time-space scales, its exchanges with the external context, its irreversible dynamics, are signs of complexity. Some concepts from the evolutionary thermodynamics, such us the theory of dissipative structures, could be extended to the city in order to investigate its behaviour. This theoretical framework suggests to analyze the city in terms of entropy and negentropy production. An emergy analysis (spelled with an "m") of an urban region is presented in order to investigate how cities maintain their organization (and decrease their entropy) by virtue of constant energy inflows from the external environment. As a result, a non-homogeneous spatial pattern of emergy density is shown as an attempt to investigate the multiple relations and energy exchanges that take place in an urban region. This approach to urban studies introduces a new energy-based vision to understand cities.