The seismic vulnerability assessment of historic masonry buildings is often constrained by limited structural data and the need for rapid evaluation tools. The first-level mechanical model proposed in Italian Guidelines for Cultural Heritage offers a simplified yet technically grounded approach for estimating the seismic demand associated with the attainment of the Limit State of Life Safety. In this approach, the capacity of a single building in terms of spectral acceleration is calculated based on a simplified evaluation of the shear resistance of the masonry walls according to the total seismic mass, and modal participation ratio, while also accounting for plan irregularities and material properties. In this study, this vulnerability model is applied to a set of representative buildings located in the historic center of Casertavecchia (Italy), a valuable medieval site characterized by unreinforced masonry. Input parameters were defined using field data and engineering assumptions, and the resulting seismic capacity was compared with the design-level demand specified by current codes. This preliminary application shows the applicability of the adopted vulnerability model to large-scale screening of historic masonry structures, also highlighting its limitations, as it is based on a rather wide range of input parameters that are not always easily obtainable from a quick geometric survey.