The reworkings of urban areas that took place during the 17th century were almost always related to overall systems of defensive fortifications, and these systems evolved steadily in terms of form and type. In response to the increased power of artillery, bastions became lower and wider, and ditches and moats were introduced on such a scale that they represented an element of transition between the city and the surrounding countryside.

Military constructions, most of all fortifications, presented one of the major expenses faced by a baroque state in Europe, and the military geography of modern Europe came into being in part because of the varying ability of states to pay their bills. The more or less constant pressure of warfare between the emerging powers of the young modern Europe induced many cities to pay for avant-garde fortifications. Excellence in planning such fortifications was originally an Italian monopoly, but it shifted northward in the wake of the conflicts.

In 1667 King Louis XIV of France began a series of aggressive campaigns against the Spanish Netherlands and the Rhineland, thus providing the opportunity to make an ambitious programme of fortifications along France’s northern and eastern borders. The pre-eminence of French military architecture results as much from the munificence of its sovereign as from the excellent qualities of its greatest military architect, Sebastien Leprestre de Vauban, who designed a series of revolutionary and ingenious fortifications, along with new cities, using ideas dictated by the pragmatic breaking of academic rules. He worked out an easy compromise between the 16th-century Italian obtuse- angle bastion and the 17th-century Dutch acute-angle bastion and reintroduced, in the Alps and the Pyrenees, the citadel, bastioned towers with blockhouses.