Castillo San Felipe del Morro, named in honor of King Phillip II, is located on a high promontory overlooking the Atlantic Ocean. It was built to act as sentinel to the entrance of the Bay of San Juan, protecting it from seaborne enemies of the Spanish crown. Construction on the largest fortification in the Caribbean began in 1539 and was completed two hundred years later. The walls, rising 140 feet above sea level and 18 feet thick, were added during the 18th century. Sentry boxes called garitas were placed in strategic positions along the walls.
El Morro and the surrounding area of 74 acres have been designated National Historic Site and World Heritage Site.
For some gorgeous photographs of Old San Juan and the fort check Janet's photo gallery.