King Charles I of Spain commissioned the first harbor defenses here in 1539, naming the fortification for King Philip II. Puerto Rico held a designation that carried enormous weight: La Llave de las Indias -- the Key to the Indies. Any European power seeking to dominate Caribbean trade routes needed to control this island, and any ship approaching San Juan Bay had to pass under El Morro's guns. The original fortress was built under conquistador Diego Ramos de Orozco, with a small proto-fortress erected in the first year to provide defense while construction continued. In 1587, Field Marshal Juan de Tejeda and Italian engineer Bautista Antonelli drew the final design -- a plan so ambitious it was part of a campaign to fortify ten sites across the Spanish Main. Construction in San Juan began in March 1589 with skilled artisans, stonecutters, masons, and smiths, alongside 150 enslaved people whose forced labor built the walls that still stand today.

El Morro's battle record reads like a chronicle of Caribbean colonial warfare. After Drake's failed assault in 1595, George Clifford, 3rd Earl of Cumberland, attacked overland in 1598 -- the only time the fortress was taken. His troops occupied it for months before dysentery drove them out. In 1625, the Dutch under Boudewijn Hendricksz slipped past the cannons and invaded from La Puntilla, sacking and burning the city even though El Morro itself held firm under Governor De Haro and militia captain Juan de Amezqueta. By 1797, the fortification system had grown formidable enough to repel General Ralph Abercromby's British invasion force of up to thirteen thousand men -- one of the largest military expeditions ever launched against Spanish territories in the Americas. El Morro's final combat came on May 12, 1898, when U.S. Navy warships bombarded the fortress throughout the day, damaging the main battery. The Treaty of Paris ended the war and four centuries of Spanish rule.

Under American control, El Morro became part of Fort Brooke, a sprawling U.S. Army post. The military filled the esplanade -- the open field of fire that had kept attackers at a disadvantage -- with baseball diamonds, officer housing, and a golf course. During World War II, the Army added a concrete bunker atop the fortress to serve as a harbor defense fire control station, watching for German submarines prowling Caribbean shipping lanes. In 1961, the Army departed and the National Park Service took over. The 1992 quincentennial restoration stripped away the palm trees and parking lots, returning the esplanade to its historic open appearance. Today over two million visitors walk the fortress each year, and a colony of roughly two hundred stray cats -- believed to descend from colonial-era or mid-20th-century rat control programs -- roams the grounds. A 2023 National Park Service plan to remove them was suspended after animal welfare groups filed a lawsuit in 2024.

Three flags fly continuously from El Morro's ramparts: the Stars and Stripes of the United States, the Monoestrellada of Puerto Rico, and the Cross of Burgundy -- the old Spanish military standard used from 1506 to 1785. The lighthouse atop the fortress, rebuilt by the U.S. Navy in 1906-08, stands 55 meters above sea level, making it the highest point on the structure and a navigation marker visible far out to sea. Puerto Rican pirate Roberto Cofresi was jailed and executed within these walls in 1825. Steven Spielberg filmed scenes for Amistad here in 1996, using the fortress to represent a slave auction fort in Sierra Leone. In that choice of setting there is an unintended truth: enslaved African laborers helped build El Morro alongside local workers, their forced labor woven into the very stone that tourists now admire. The fortress endures as monument, museum, and reminder -- beautiful, imposing, and honest only if you listen to the full story of who built it.