The term resilience generically indicates the ability of a system to maintain its functionality despite the occurrence of catastrophic events. The problem of numerical quantification of this capacity naturally leads to the search for a synthetic index, which can suggest whether and how to act to prevent the system from suffering substantial damage with the occurrence of unpredictable events. In the presentation, a particular resilience index for mechanical structures will be introduced based solely on their kinematic and mass arrays, as well as on the most pessimistic scenario. Porta Maggiore—in Rome—lends itself as an opportunity of application for experimentation and allows to test how the index can be applied to cultural heritage sites. The study of the monument’s original geometry, as well as its phases of transformation and inclusion of the Claudian aqueduct arches in the Aurelian wall, help to define the levels of resistance the structure can oppose to different stresses. Furthermore, it is desirable that such experimentation could lead to predictive analyses on mechanical responses to exceptional events. Within the framework of a multidisciplinary study, the definition of the resilience index is supported by a whole knowledge of all structural characteristics of the architecture, as an essential data base to make what is proposed even more effective and useful for a real preservation of archaeological buildings.