The structural analysis of an about 30 m tall building is presented in this paper. It was constructed in 70s, so its use pointed out the issue of the seismic reliability of existing buildings, especially if considered of strategic importance for civil protection. The structure is composed by two parts, the lower one in reinforced concrete, which contains a theatre room, and an upper in steel. The study allowed defining the seismic vulnerability. A refined finite element model was assembled and updated on the basis of comprehensive experimental analyses on materials and structure and also of experimental modal analysis, highlighting the dynamic effect due to the irregularity of the building and the influence of the adjacent building, the seismic joint is missing. The results pointed out a quite high vulnerability of the building to seismic actions and the inadequacy of the pile foundation to support horizontal seismic loads. A retrofit intervention, based on the insertion of a seismic isolation system and the realization of a seismic joint with the adjacent building, is proposed. It allows reducing the horizontal forces in the piles and the torsional movements of the structure.