Metal rods and prestressed cables can be used for the strengthening of masonry structures and monuments. These techniques are partially reversible and introduce minimal changes in the characteristics and the structural system. Therefore they are practical and economical alternatives to more traditional strengthening methods. The research to date has mainly focused on component behavior and only few results about the effectiveness of post-tensioning retrofits on overall structure response has been published. In this paper a number of numerical models and extensive parametric investigation was done in order to study the effectiveness of the previously mentioned technique, by analyzing the response of a masonry tower as a case study. Both static and dynamic analyses considering base excitation loads were performed. The effectiveness of the prestressing depends on the location of the cables, the material and the geometry of the structure. Prestressing must be accompanied with local reinforcement in order to reduce excessive deformations and damages.