A construction history that spans centuries often results in complex structural systems whose load-carrying behavior cannot be easily assessed by visual inspections. Their structural rehabilitation should be supported by numerical analyses during both the design and the implementation phases of a retrofit operation. Within this context, monitoring systems provide reliable information about the structure performance under serviceability conditions. When outputonly measurements are available for the dynamic system identification before and after the intervention, signals processing techniques can be applied to assess both the original deteriorated state of the structure, and the relative efficiency of the retrofit operation. The procedure is illustrated with reference to a specific case study, which concerns a part of the Archbishop Palace located in Siracusa, Italy. Experimental and numerical studies are carried out to evaluate the actual loads supported by an ancient stone arch. The arch is surmounted by a building which underwent several additions in elevation across the centuries. Experimental tests are performed using different excitation methods and accelerometer configurations. The measurements are taken before and after the retrofit of the arch. The elaboration and comparison of the collected data for the structural assessment of the system before and after its retrofit is the topic of this work.