During the 16th century, recognizing the need to protect the Spanish treasure fleets on their voyages to and from the New World, the Spanish erected vast fortifications throughout their territories in the Caribbean Islands and the Gulf of Mexico. Designated a World Heritage Site, the Spanish system of fortifications in San Juan, Puerto Rico is the oldest European construction in territory of the United States and one of the oldest in the New World. These fortifications guarded the entrance to San Juan Bay, helped the Spanish maintain sovereignty over Puerto Rico, and protected Spanish commerce in the Caribbean basin. The forts and three miles of city wall are fine examples of military architecture reflecting the power and glory of the Spanish Empire and the beginning of European ascendancy in world affairs. San Juan National Historic Site preserves these massive fortifications and offers youth-focused programs on the Spanish and Latino history of the island forts at its Discovery Center.

Casa Blanca and La Fortaleza were the first fortifications the Spanish constructed in San Juan after Columbus reached Puerto Rico in 1493. Erected in 1525, the Casa Blanca was a small blockhouse the Spanish settlers built during the first years after San Juan’s establishment in 1521. The house provided defense against attacks from the Caribbean Indians and a home for the Puerto Rican governor’s family, who at the time were the descendants of the Spanish explorer Juan Ponce de Leon.

The Spanish built the second stronghold, La Fortaleza, also known as the Santa Catalina Palace, from 1533 to 1540. The original fort included a circular tower and four massive stonewalls surrounding a large patio. The circular tower has thick stonewalls and two vaulted rooms, one of which is the chapel.  The tower is called “Torre del Homenaje” or “Tower of Homage,” which came from the custom of the governor to make a solemn oath of loyalty and courage at the top of the tower in times of danger. The Spanish military added a second tower at the end of the 16th century. La Fortaleza is a fine example of Spanish architectural style, which is characterized by heavy-tiled flat roofs, sunlit patios, galleries, casements, graceful arches, grilled doorways, wrought iron, tile, and mahogany staircases.

Front view of La Fortaleza from street in San Juan, Puerto Rico Courtesy of VXLA, Flickr's Creative Commons

Captured first in 1598 by the English and then burned by the Dutch in 1625, La Fortaleza provided only minimal protection for the port of San Juan Bay. After the Dutch burned La Fortaleza, the fort was rebuilt and enlarged, and by 1640, it was expected for the Governor of Puerto Rico to reside at La Fortaleza. On November 27, 1822, an official decree established La Fortaleza as the residence of the Governor of Puerto Rico, thus making official a custom observed since the 17th century.

In 1846, La Fortaleza underwent a massive reconstruction to change its military appearance into a palatial façade, but the original “Tower of Homage” still stands. Today, La Fortaleza is the oldest executive mansion still in use as such in the Western Hemisphere. La Fortaleza is recognized as a National Historic Landmark within the San Juan National Historic Site.

Recognizing their mistake with the positioning of La Fortaleza, the Spanish began construction of a stronger and more efficient fortification for the protection of the city’s anchorage at the entrance of the San Juan harbor in 1539. Castillo de San Felipe del Morro has a round masonry tower and resembles a castle. By 1596, El Morro’s strategic location and four-cannon defense system proved successful, after it withstood the first British attack led by English privateer Sir Francis Drake. Although the Spanish defeated Drake’s forces, the English Crown was determined to seize Puerto Rico, and in 1598, commissioned the Earl of Cumberland to lead the second attack on San Juan.

Cumberland realized Drake’s mistake in attacking the Spanish fortification from the harbor entrance and instead besieged El Morro from behind through a successful land attack. Eventually, the Spanish regained control of the fort after an epidemic forced the English to abandon El Morro, which the Spanish rebuilt to protect the fort from future land approaches. In 1625, the fortification withstood the Dutch invasion, but was unable to protect the city, which the Dutch burned down on their way out of San Juan. Following the attack, the Spanish spent the next 150 years building a defense line around San Juan to protect the city and its people from enemy attacks. Completed in 1678, the 50-foot high wall surrounded the city that stretched from El Morro, bordering the city on both sides, and closing in at San Cristobal.

Castillo de San Cristobal, which dates from 1634, was part of the new defense line to protect San Juan against land approaches coming from the east.  The Spanish erected the fortification on the northeastern edge of San Juan, which was 150 feet above sea level and a mile away from the headland where El Morro stood.

The smaller Fort San Juan de la Cruz (1606), which lies across from Fort San Felipe del Morro, had the strategic purpose of providing crossfire to El Morro and additional protection of the entrance to the harbor.

“The First Flash of Fire and Smoke,” of the Spanish American War Courtesy of the National Park Service

In 1678, King Charles III--who had designated San Juan as the defender of the first order--sent two Irishmen to strengthen the city’s fortifications and defense lines. By the end of the 1780s, Thomas O’Daily and Alexander O’Reilly had rebuilt El Morro, San Cristobal, and the defense wall into its present day image, transforming San Juan into the most powerful stronghold in America.

With the new upgrades, San Cristobal became the largest Spanish fortress in the New World, with 450 canons, and expanding over 27 acres of land. The Irish engineers designed the defense system on the principle of “defense in depth,” which meant that enemies would have to break several barriers--that were each higher and stronger than the one before —to besiege the fort. In 1797, the in depth defense design of San Cristobal proved successful, when Sir Ralph Abercromby’s British forces were unable to seize the city after a failed attempt to break the first barrier. Although the British attacked the fortification, San Cristobal never fired a shot, proving the strength and effective engineering that went into building the fort’s barriers. For 100 years, San Cristobal would not fire a single shot; it was not until the Spanish American War in 1898 that one of the fortification’s guns fired the first shot into the Atlantic. 

At the conclusion of the Spanish American War in 1898, after the United States gained control of Puerto Rico, the forts of Old San Juan continued to serve their military purpose by defending the American territory until the end of World War II. Today, at San Juan National Historic Site, the National Park Service protects the forts for visitors to explore.