The geometry underlying the design of sixteenth-century fortifications is a subject of considerable interest among scholars of military architecture. Many treatises from the period emphasize the importance of using geometrical constructions for the design of defensive elements in proper proportion to the entire edifice. One treatise, the Epitomi delle fortificationi moderne, was written in 1560–1561 by Italian military engineer Giovanni Battista Antonelli (1527–1588), whose family had been in the service of the Spanish crown for nearly a century. In this paper, we investigate the methods set forth by Antonelli in his work, paying particular attention to the geometrical design of typical architectural elements in fortifications of the early modern period. We then provide a case study of the reconstruction of a bastion according to the engineer’s proposals.