In the seven Centuries after its foundation, the Order of the Knights Hospitaller of St. John, said at thebeginning of Jerusalem, of Rhodes and finally of Malta, reached a major economic power and a stronghierarchical structure based on the articulation of national sections. Maltese fortifications erectedbetween 1530 and 1798 are in some respects unique; they nevertheless are a representative example ofcontemporary European military architecture at its best. What is very interesting is the diversity ofsocial and professional categories of military engineers, already committed by the Pope or by the bigdynastic families of the time, called to work for the Hospitaller Order.This text tries to investigate the relationship between historical events and their transformations that ledto the configuration of Valletta, permeated by theoretical concepts and studies introduced in Malta byFrancesco Laparelli, military engineer from Cortona, as aspects of military defense, the design of a cityand architectural principles. Theory and practice converge in the new city of the Order, in a milieu quitedifferent from the historical context of the Sixteenth Century.