Castillo San Felipe del Morro (English: Promontory Castle of Saint Philip), most commonly known as El Morro (The Promontory), is a large fortress and citadel in the Old San Juan historic quarter of San Juan, the capital municipality of Puerto Rico. Commissioned by King Charles I of Spain in 1539, it was first built as a fortified tower in honor of King Philip II, who oversaw its expansion into a hornwork bastion fort by 1595. Over the next 200 years, especially in the reign of King Charles III, El Morro continued to be developed to reach its current form in 1787. Rising 43 metres (140 ft) from the Atlantic shoreline with 5.5 to 7.6 metres (18 to 25 ft) thick walls, the six-leveled edifice stands on a steep, rocky headland promontory on San Juan Islet guarding the entry to San Juan Bay, the harbor of Old San Juan. El Morro, alongside La Fortaleza, San Cristóbal, El Cañuelo, and other forts part of the Walls of Old San Juan, protected strategically and militarily important Puerto Rico, or La Llave de las Indias (The Key to the Indies), from invasion by competing world powers during the Age of Discovery and Sail. It was designated a World Heritage Site by UNESCO in 1983.

Lying on the northwesternmost point of the islet of Old San Juan, Castillo San Felipe del Morro is named in honor of King Philip II of Spain. The fortification, also referred to as el Morro or 'the promontory,' was designed to guard the entrance to the San Juan Bay, and defend the Spanish colonial port city of San Juan from seaborne enemies.

In 1983, the citadel was declared a World Heritage Site by the United Nations in conjunction with the San Juan National Historic Site. Over two million visitors a year explore the castillo, making it one of Puerto Rico's leading tourist attractions. Facing the structure, on the opposite side of the bay, a smaller fortification known as El Cañuelo complemented the castillo's defense of the entrance to the bay.

The construction of the citadel and its surrounding walls began in 1539 on orders of King Charles V of Spain. The original fortress was built under the direction of conquistador Diego Ramos de Orozco and its main purpose was to defend the port of San Juan by controlling the entry to its harbor. In order to have a viable defense while the rest of the fort was being completed, a small proto-fortress was erected during the first year of construction. It is estimated that this section comprises about 10% of the whole structure.

It was not till 1587, however, that Field Marshal Juan de Tejeda and the Italian engineer-architect Bautista Antonelli drew the fort's final design. The plan, which was based on the then firmly established Spanish military fortification principles of the time, included fortifying nine other sites in the Spanish Main and Spanish West Indies: Santo Domingo, Santa Marta, Cartagena, Nombre de Dios, Portobelo, the Chagres River, Panama City, Havana, and St. Augustine. San Juan construction began in March 1589 with skilled artisans, 12 stonecutters, 18 masons, 2 smiths, a cooper, metal founder, and an overseer assigned to the task, with the help of 150 slaves. Captain General Diego Menéndez de Valdés, who was the governor of Puerto Rico, took over construction after Tejeda and Antonelli left to start construction of Santo Domingo's fortifications. Capt. Pedro de Salazar took over construction in 1591.

The new fortifications consisted of a hornwork, crossing the headland, to protect the landward side of the existing tower and water battery. Two half-bastions, one on the Atlantic side called "Tejeda", and another on the harbor side called "Austria", were connected by a curtain wall fronted by a moat, and spanned by a drawbridge in the center. The gate and drawbridge were protected by a ravelin, and just inside the gate was a guardhouse. A pair of batteries behind the hornwork overlooked the sea and harbor.

In 1634, construction of the city walls surrounding San Juan began. By 1650, the town was enclosed on the east, south and west, while natural battlements protected the city along the Atlantic.

In 1765, Alejandro O'Reilly, Inspector General of Cuba, and Colonel Tomás O'Daly, San Juan Chief of Engineers, agreed on a plan to strengthen San Juan's defenses, which was approved by Charles III of Spain. San Juan became a Defense of the First Order, and one of the most powerful plazas in the Americas by 1790. El Morro's improvements included three cisterns under the main plaza containing 820,000 litres (216,000 US gal) of water collected from times of rain. The walls were strengthened to be 5.5 to 12.2 metres (18 to 40 ft) thick. These walls consisted of limestone and sandstone blocks forming the exterior and interior, with rubble sandwiched in between. The Santa Bárbara battery became the main battery with 37 cannon, supported by casemate guns at a lower level, and backed by a great wall with casemates at a higher level.