The age of Italy’s building heritage, combined with the impact of recent severe earthquakes in central Italy — which caused extensive damage and collapse — has led to a growing focus on seismic vulnerability assessment and the preservation of historic structures. Monumental masonry buildings, especially those in Italian historic centers, are often complex in form, having undergone architectural evolution over time and comprising portions built in different historical periods. As a result, assessing their seismic vulnerability is a highly challenging task. One common characteristic of these buildings is the addition of structural portions over time, which is also typical of aggregate buildings — structures built in direct continuity with one another, sharing boundary walls. However, unlike aggregate buildings, complex masonry buildings cannot be treated as simple aggregations. While they also result from successive additions, they tend to maintain a certain structural unity, distinguishing them from the more fragmented nature of urban aggregates. This study investigates the seismic vulnerability of the Garibaldi Barracks in Naples, a representative case of a complex masonry building. Over time, the structural complex has evolved significantly. Originally constructed in the mid-19th century for military purposes, it incorporated the 14th-century convent of the Augustinians of San Giovanni a Carbonara, along with two Renaissance towers that originally linked the northern and eastern defensive walls. These towers were later renovated and vertically extended. The wall segment between the two towers was replaced with a new construction, now forming the main façade of the barracks. To assess the seismic vulnerability of this complex structure, a nonlinear static (pushover) analysis was conducted under two limiting hypotheses: (1) the building is considered as a standalone structure, neglecting interactions with adjacent constructions; and (2) the entire complex is analyzed as a single structural unit, accounting for interaction effects between the various building portions.