Fuerte San Felipe del Morro

Castillo San Felipe del Morro, most commonly known as El Morro, is a large fortress and citadel in the historic district of Old San Juan, Puerto Rico. Commissioned by Charles I of Spain in 1539, it was first built as a fortified tower in honor of Philip II, who oversaw its expansion into a hornwork fort by 1595. Over the next 200 years, especially in the reign of Charles III, El Morro continued to be developed to reach its current form in 1787. Rising 140 ft from the Atlantic shoreline with 18 to 25 ft thick walls, it 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, La Llave de las Indias, from invasion by competing world powers during the Age of Sail.

Source: Wikipedia