In the early nineteenth century, the (obsolete) Portuguese urban fortifications were frequently ruined and suffering from a gradual desertion around them. With the emergence of patrimonial concerns, those defensive structures began being considered as historical and cultural monuments. As a result, the isolation of these monuments became a common practice that often created public green areas (gardens and parks) framing monuments, giving them a picturesque image. The consequences of these rehabilitation actions on urban fortifications in Portugal are analyzed here, focusing particularly on those that created new public green spaces within and around urban areas. In fact, the shape of several Portuguese cities was conditioned by those interventions: not only were breathing areas conceived in the middle of dense urban masses, allowing their fruition by local populations, but also some ancient urban belts associated with former defensive needs were recovered by those interventions.