Structural and dynamic identifications of historical buildings provide key input parameters in present archeo-seismological research. The ambition of the research project ACROSS (ArChaeology, inventory of RecOnstruction, Seismology and Structural engineering), involving different institutions in France and Italy, is to study past earthquakes through their impact on historical masonry buildings. Within this context, and among the first tasks, an ambient vibrations recording campaign was carried out to characterize five ancient bell towers in the Mugello basin in Tuscany (Italy) that suffered damage from at least one strong earthquake during medieval times. Operational Modal Analysis (OMA) allows us to identify the five slender masonry towers' main vibration modes (frequencies, modal shapes, and damping coefficients). Mode shapes exhibit the singularities of the bell towers related to their geometrical complexity, the use of numerous building techniques, and the relationships with other structures, particularly the adjacent naves. Of particular interest is the study of the response of one of the bell towers (pieve di St Cresci in Valcava) to bell-swinging excitation that has given insight into its non-linear dynamic response. Apart from helpful information about the relation between slenderness, irregularities in height, and main resonance frequencies, the results of the OMA study serve as the basis for building numerical models of slender structures that will help understand past (and future) dynamic behavior under different earthquake scenarios.