Through the conquest of Mexico and Perú, the Spanish Empire obtained great riches in jewels, gold and silver. Each year, two fleets of armored galleons set out from the other country for the New World for the purpose of bringing back the treasure to the "safe harbor"....Spain!

    The sailing ships (galleons) were the only mode of transport and the winds the only source of "fuel". Favored by the trade winds which blow constantly from Africa towards the islands of the Caribbean the two Spanish fleets, one bound for Vera Cruz and the Philippines, the others bound for Portobello, in Panama, would travel along the coasts of Puerto Rico.

    For these treasure-laden galleons, the Caribbean sea was a route of vital importance but also, one of danger. Spain had declared the Caribbean an "exclusive territory", which provoked attacks by both pirates and the nation’s traditional enemies. The attacks and the ever-present threat of tropical storms, together with the strategic location of the island, caused the deep harbor of San Juan to become a "safe haven" from which to escape these dangers. This was one of the reasons which caused the Spanish Crown to see the necessity of fortifying the city of San Juan and its harbor, converting it into a strong and strategically placed naval base which would enable them to control the sea lanes and the access to the Indies. The other reason was a matter of military security, protecting San Juan and its harbor from a possible invasion which would provide an enemy with a base from which to attack Spanish ships and settlements. In consequence the King of Spain, Philip II, ordered the building of the fortifications and designated the city "The Key to the Indies".