This paper presents the results of the performance assessment of an existing masonry tower and the subsequent repair and strengthening intervention. The study analyses an Italian architectural heritage building in the Cilento National Park (Southern Italy): the bell tower in Torre Orsaia. On-site diagnostic investigations including geometrical surveys, laser scanning surveys, flat jack tests, endoscopic tests, sonic pulse velocity tests, geognostic surveys, and tie-rods tests were described in detail. The multilevel assessment path proposed by the Italian Guidelines for the assessment and mitigation of the seismic risk of the cultural heritage was followed and the corresponding results were discussed. The local collapse mechanisms were studied through a kinematic limit analysis based on rigid block rotation. The global inelastic behavior of the masonry tower was studied through a macro-model approach using the nonlinear static (pushover) analysis. The repair and strengthening intervention showed their effectiveness to repair the existing cracks, preventing the damage to the belfry, and improving the seismic performance of the tower. Both the compatibility, durability, and reversibility of the interventions and their reliability and monitoring were finally highlighted since the bell tower of Torre Orsaia is a historical heritage building.