San Juan de Ulúa occupies a reef and small island beside the harbor of Veracruz. In 1518 the expedition of Juan de Grijalva reached the place, combining the Christian feast name San Juan with a European rendering of Culúa. Hernán Cortés arrived the following year and founded the political community of Veracruz before beginning the invasion of the Mexica Empire. The anchorage became Spain’s principal Atlantic gateway to New Spain, linking the colonial capital with Caribbean and European routes.

Early construction served shipping as much as defense. Workers created docks, mooring walls, warehouses, and shelters where passengers and cargo could transfer between ocean vessels and the mainland. Silver, cochineal, hides, cacao, textiles, enslaved people, officials, soldiers, and correspondence passed through the port. Because treasure fleets gathered there before crossing the Atlantic, the harbor attracted privateers and rival imperial powers. Storms, corrosive salt water, shifting foundations, and disease made construction and service exceptionally difficult.

In 1590, under orders issued during the reign of Philip II, engineers began developing San Juan de Ulúa as part of a wider defensive system for Spain’s American ports. The complex expanded over generations rather than through one campaign. Curtains, bastions, towers, vaulted rooms, cisterns, and protected docks formed a low fortress adapted to artillery and the reef. The devastating pirate attack on Veracruz led by Laurens de Graaf and Nicolas van Hoorn in 1683 encouraged further modernization of the port’s defenses.

The fortress played a decisive role after Mexico declared independence. Spanish troops withdrew to San Juan de Ulúa and used it as their final stronghold on the mainland, at times bombarding Veracruz. Short of supplies and weakened by disease, the garrison capitulated in November 1825. Its surrender ended the last sustained Spanish military occupation in the new republic. The site later came under fire during France’s Pastry War in 1838 and the United States invasion of 1847, demonstrating the harbor’s continuing strategic value.

San Juan de Ulúa also became notorious as a prison. Colonial and national governments confined political opponents, military prisoners, and people convicted of ordinary crimes within damp, poorly ventilated chambers. Stories surrounding prisoners such as Fray Servando Teresa de Mier and the bandit Chucho el Roto contributed to its grim reputation, although legend sometimes obscures the exact record. The fortress again figured in the United States occupation of Veracruz in 1914 and in revolutionary-era struggles over control of the port.

Changing harbor works eventually joined the former island more closely to the mainland and reduced its military role. Preservation has had to confront water intrusion, salt crystallization, settlement, storms, and the cumulative effects of centuries of alteration. Archaeology has also revealed earlier construction hidden within later defenses, clarifying how the complex gradually grew. Under the care of Mexico’s Instituto Nacional de Antropología e Historia, the fort became a museum and archaeological monument. Its walls preserve the history of first contact, colonial trade, imperial defense, independence, foreign intervention, punishment, and modern conservation at the maritime entrance once called the key to New Spain.