On April 14 of 1579 Felipe II declared, by Real Certificate, Acapulco as the only commercial port between America and Asia.  The Port was intended to receive and dispatch the Galleon of Manila, as well as the few ships originating from Peru, Chile and Central America.

The Consortium supporting the trade requested that several Viceroy protect the Port from the pirates. Diego Fernández de Cordoba commissioned the Dutch engineer, Adrian Boot, to accomplish this fortification that's characterized by its irregular polygon.  Building began in 1615 and was completed in April of 1617.

Due to an earthquake in 1776, the original Fort of San Diego collapsed.  A new Fort was built in 1783 and lasts to this day.  The building is unique for its design, applying architectural concepts advanced for this era.  As such, it's a master work of military engineering for the times. Its pentagonal shape permits self-defense on all flanks. It is, furthermore, surrounded by a dry pit. The Fort housed two thousand men, containing provisions and ammunitions for a year.  There is also an efficient system to capture, concentrate and preserve the rainwater and for numerous domed rooms about the central courtyard.

At the beginning of the Independence War, the Fort or Castle of San Diego, provided the stage of one of the more notable war actions; Don Miguel Hidalgo's order to the General Morelos to organize the campaign of the South and taking of Acapulco in 1811.   Having fulfilled this, the "Commander of the South" Emiliano Zapata, uttered the famous phrase: Viva España , hermana, no dominadora de America (Long live Spain, our sister, not dominated by America!)
After that it fulfilled its assignment as custodian of Acapulco and of the Galleon of Manila.  The fort was assigned various uses, until in 1986 it became the headquarters of the Historical Museum of Acapulco.