Bastión de San Fernando

This was a projecting part of a fortification built at an angle to the line of the wall so defenders could fire at their enemies in multiple directions in San Felipe del Morro. This structure is located on the left side, outside el Morro.  

Source: The Forts of Old San Juan: San Juan National Historic Site, Puerto Rico. Washington, D.C.: Division of Publications, National Park Service, U.S. Department of the Interior, 1996.