Early Modern military engineers are quite obvious ‘shadow agents’ of war: not necessarily present on the battlefield, their impact on the art of war was nevertheless considerable. The complexity of the professional profile of Renaissance military engineers during the ‘military revolution’ still makes their identity a historical riddle. In this chapter, I will try to address two issues concerning Renaissance military engineers from the standpoint of the history of science and technology: the cultural models behind their apparent polymathesis – here intended as a wide-ranging learning freely pursued independently from cultural models – and their agency in lesser-known military affairs such as technological propaganda, intelligence and astrology.