In recent years, we are experiencing a digital revolution in many contexts, the digitalization of industry (Industry 4.0), the digitalization of public administrations, the digitalization of people and things (Internet of Things) and, of course, the digitization of cities under the name of Smart Cities. Although the digitization process is based on 3 main pillars: data, talent and innovation. It is necessary to make a mixture of these 3 pillars in different percentages to achieve digitization. In the case of cities, great weight lies in the technology deployed that makes up the architecture of the city. This technology is very incipient and lacks standards that allow interoperability between different devices. This article tries to show a picture of the possible architectures existing today from the point of view of sustainability and energy efficiency.