The Spanish Crown early perceived the strategic nature of Ghilets southernmost region which, through the Magallanes Strait and Cape Horn, was an intermediate point for navigation from Europe to the American Pacific coast. From the end of the XVI century, the construction of fortifications in this area became a high priority due to the frequent transit of French, English and Dutch ships and, particularly, to pirates' raids. To the destruction in the Chilean and Peruvian coast caused by Francis Drake in 1578, were added expeditions like that of the Dutch Hendrik Brouwer, who, in 1643, occupied for a time the littoral adjacent to the mouth of the Valdivia river, with the purpose of challenging from there the Spanish might.
These incidents led the Spanish authorities to undertake the construction of powerful defensive facilities in Valdivia, Valparaiso and Chiloé. Valdivia, the oldest of the three, wouid become, along with El Callao, the most important defensive complex of the American South Pacific coast. Both are exceptional samples of the Hispanic-American school on fortifications. The reason for this effort and the resources invested in Valdivia stem from the need of defending Peru, colony which along with Mexico constituted the main source of wealth of the Spanish Crown.
In 1645, the Viceroy of Peru, Antonio de Toledo, Marquis of Mancera, started the execution of a patiently designed defensive plan. One of its fundamental aspects was the dispatch of a great Armada to refound the city of Valdivia -devastated as a consequence of the 1598 native revolt-, and the raising of fortifications on the coast. The contingent in charge of the mission was organized in Peru, and it astounded its contemporaries by its magnitude. Seventeen ships were gathered, equipped with an amount never seen before of building materials and supplies.
The original fortification plan was based on using the exceptional defensive qualities of the Corral Bay, at the mouth of the Valdivia River. The project contemplated the building of four basic fortresses which, in case of attack, wouid jointly operate with its cross-fire. In the disposition of these four central points and in the design of these bastions, topographical, geographical and environmental factors were fitted together: sea currents, ground unevenness, prevailing winds, etc. Although with the passage of time the installations grew with regard to the number of batteries, and the role of the four basic fortresses changed, the original scheme remained unaltered, with four sites retaining their leading part: the Mancera island, Corral, Amargos and Niebla.
The main bastion of this defensive complex was the Constantino island, later called Mancera. The island is located in the middie of the bay into which the Valdivia river flows, and there the San Pedro de Alcantara Castle was built according to the project designed by the Navy chief engineer, Constantino Vasconcelos. The castle, made of stone, was armed with fifteen pieces of artillery, and had a moat and two turrets. Inside it, among other installations, there was a church and two convents: one Franciscan, an the other Augustinian.
At the so called Punta de Amargos, on the southern side of the Valdivia river mouth, the San Luis de Alba Castle was built, entirely of stone. It had up to eleven pieces of artillery, which due to its strategic position, couid batter down the anchorage of enemy ships. The castle was isolated from it surroundings by a moat, crossed by a draw-bridge. Aside from the quarters and the commander's house, there was a chapel inside the castle. At the end of the XV111, the bastion was reinforced, and some buildings in brick were added to it. Nowadays, none of the inner constructions of the complex remain standing, yet the basic stone structure with its artillery pieces has survived and has been restored.
The Fort of Niebla stands up on the northern side of the Valdivia river mouth. It was built on a cancagua stone slope about 30 meters high, dominating the entire bay and open sea. Its very original design adapts itself well to the site's geography. The Fort of Corral, to the south of the said mouth, was fully remodeled in the second haff of the XVIII century. It consists in a long battery facing the sea: 24 cannons standing on a solid stone wall. Both the inner constructions and the defenses towards inland have vanished.
In the second half of the XVIII century, a thorough plan was carried forward for restoring and improving the fortresses. The engineers José Birt and Juan Garland were commissioned for the task. The defensive complex of Valdivia reached the point of having 17 bastions, what with surveillance installations, castles, fortresses and batteries. This complex wouid exert during the Colony a thoroughly efficacious deterrent effect, since, in fact, it frustrated the raids of the rival powers. Paradoxically, those who took these defenses to pieces weren't the European enemies, but the independentist patriots.