In the present paper the experimental and numerical activities related to in-situ Ambient Vibration Tests performed on a monumental masonry building placed in Castelnuovo of San Pio (Italy) are presented. The obtained experimental results, able to identify the dynamic characteristics of the investigated palace, have been successively reproduced by means of a building FEM model through the ABAQUS calculation code. Numerical frequency analysis has been also performed on the original building aiming at evaluating its reduction in stiffness due to earthquake. Finally, appropriate retrofitting interventions have been proposed, their effectiveness being proved by numerical analyses on the improved palace FEM model.