Military engineering is another major theme in the work of Taccola and Francesco di Giorgio. Taccola was seeking employment with Emperor Sigismund of Hungary, who visited Siena in 1432-1433.
Francesco, meanwhile, collaborated with two of the leading warrior-princes of the fifteenth century: Federico da Montefeltro, Duke of Urbino, and Alfonso of Aragon, Duke of Calabria.
Taccola and Francesco were living in a transitional period for military technology. The devices and techniques of classical and medieval times were still in widespread use, while the advent of firearms and the bombard - defined by Francesco di Giorgio as a "diabolical invention" - posed an ever-greater threat.