The structural analysis of monumental constructions requires considering safety and conservation objectives, including the possible presence of artistic assets. In order to face these issues, this paper presents the results of a diagnostic analysis carried out on a 14th-century fortress: the Rocca Albornoziana of Spoleto in Umbria. Within this context, particular attention was de-voted to the choice of the most reliable modelling strategy for the application of the displacement approach in the seismic Performance-Based Assessment (PBA) procedure, as a function of different possible seismic behaviors. Seismic vulnerability was evaluated using a pushover method, and the results obtained with the nonlinear numerical model have been compared with the simplified schemes of the limit analysis. The capacity of the fortress to withstand lateral loads was evaluated with the expected demands resulting from the seismic action. By comparing the seismic demand with capacity, the high vulnerability to extensive damage and collapse, as frequently observed in similar buildings, was demonstrated. The aim of this study was to take into account both the qualitative, related to the technology of construction and to the rule-of-the-art, and the quantitative aspects of the numerical analysis of the structural load-bearing elements.