With the sixty-year peace of the Iberian Union, between 1580 and 1640, of a king for the crowns of Spain and Portugal, the coastal castles maintained their importance and supremacy over the border castles, as danger lurked daily on the coast due to incursions by squadrons, corsairs and ships from the kingdoms with which Spain was in conflict, be they English, Dutch or French. The military reinforcement focused on the programmatic lines that had been in place since the Middle Ages, with the river routes of the Lima, Douro, Mondego, Tejo and Sado rivers as a defensive strategy, which had a strong influence on the regions where walled centers of royal power and the Military Orders of the Temple, of Saint John of Jerusalem, of Santiago and of Christ were built.
In addition to the defensive reinforcement of some strategically built structures on the Portuguese coastline and over time, such as the Citadel of Peniche and the Fortress of São Julião da Barra, in Oeiras, there was even a need for new constructions, such as the Fort of São Filipe, in Setúbal, or the Fort of São Lourenço do Bugio, on the islet of Cabeça Seca, in the center of the River Tagus, in Lisbon.
Although the coastal defense system was earlier, it was with the acclamation of the House of Braganza as kings of Portugal, in 1640, that numerous bastioned constructions were carried out, in order to consolidate the defense of the coast as a whole and also at strategic points, often isolated but easy for invading armies to disembark and enter the territory. This new strategic thinking led to the construction, without European parallel, of a remarkable set of forts during the 17th and 18th centuries, often supported by citadels where the bulk of the armed forces were located, not only in mainland Portugal, but also in the Atlantic space, in the East, in Africa and in Brazil.
