Evidence of damage induced by past and recent earthquakes in Italy testifies to the seismic vulnerability of masonry towers, especially when founded on soft soils. A significant case study is represented by the 68 m-high Carmine Bell Tower, located in the coastal area of Napoli. The structure, the foundation and the subsoil of the tower were thoroughly investigated through field and laboratory tests and dynamic monitoring. Nonlinear seismic response analyses of the soil–foundation–structure system were carried out on a full dynamic 3D model, for which the parameters were accurately calibrated from experimental data. Because the tower stands between a church and a friary, the effect of the lateral constraints exerted by the neighbouring structures was numerically investigated. The lateral constraint imposed along the tower was found to strongly affect the seismic response of the system, as it changes the position and the time of occurrence of the plastic zone that takes place in the structure or the foundations. Finally, the foundation safety was assessed by comparing the evolution of the base forces to the failure locus.