A simplified 3D CAD-based methodology for the rapid assessment of the seismic vulnerability of existing masonry towers is introduced. This approach relies solely on the detailed 3D geometric model of the structure and automatically computes the collapse acceleration for a user-defined failure mechanism. The study investigates a limited set of predefined mechanisms, including vertical splitting, base overturning, rocking with inclined yield lines, and combined rocking and vertical splitting. The restriction to these mechanisms is informed by prior numerical studies and observations from post-earthquake surveys. However, users have the flexibility to define custom mechanisms tailored to their case studies by directly modelling distinct volumes within the CAD environment. The process is fully automated, employing the principle of virtual work under the assumption that masonry behaves as a no-tension material. This enables a direct calculation of the horizontal collapse acceleration. According to the kinematic theorem of limit analysis, the mechanism associated with the minimum acceleration is the one most likely to occur during a seismic event. This streamlined approach facilitates seismic vulnerability assessments of masonry towers and is particularly accessible to practitioners without expertise in advanced finite element analysis or limit analysis, making it suitable for diverse users involved in cultural heritage preservation. To demonstrate its capabilities, the automated procedure is applied to a historical tower in Italy. Additionally, the approach has been validated through scaled 3D-printed physical models tested on a tilting table to verify the accuracy of the results, as well as through an advanced finite element model implemented in Straus7.