In this paper I will discuss how open-air spaces were used in the Roman cityscape, especially in Rome, Pompeii and Ostia, and what was the role of these spaces as dividers or connectors of the urban landscape. Such spaces may have had flexible or alternating uses during the centuries; they may be associated with the relocation and redefinition of urban activities, as well as with the expansion of a city, but they may also be connected to urban decay, abandonment of various parts of the cityscape, or to the deconstruction of the cities in phases of decline. Examples of the development of the urban areas and activities are presented in these three cities that were chosen as exempla, comparing material and literary evidence consisting of Roman literary sources, inscriptions and the results of archaeological excavations. An important aspect in this context is also connected to the placement of the sanctuaries and the necropoleis of the cities as compared to the city-walls.