Terreplein of the curtain wall, part of Castillo San Felipe del Morro’s Cavalier Battery, San Juan National Historic Site

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El Morro Hornwork Main Page

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In the 1770s and 80s, Castillo San Felipe del Morro (aka El Morro) received a major overhaul by the Spanish military. The walls and terrepleins (artillery decks) of the fort’s hornwork, which is comprised of the Ochoa and Austria bastions and the wall between them (curtain), were raised approximately eleven feet. On the ocean side of the fort, the back retaining wall of the Santa Barbara Bastion was thickened and also raised in height to match the hornwork. It henceforth became known as the Great Wall.

Hornwork of Castillo San Felipe del Morro, San Juan National Historic Site

Great Wall and Cavalier Battery of Castille San Felipe del Morro

Casemates—fortified rooms with arched ceilings that are typically designed to hold artillery—were installed on the Plaza de Armas, the main plaza between the curtain and Great Wall. The roofs of the casements formed terrepleins for both walls, allowing artillery to be mounted around the entire upper level of the fort. The structure, which forms a loop around the plaza, is now known as the Cavalier Battery. Cavalier is the term used for the highest structure of a fort.

Typical casemate in a military fort

Cavalier Battery of Castillo San Felipe del Morro

Upon entering El Morro, you will be on the Plaza de Armas. To access the Cavalier Battery, head towards one of the archways at either end of the plaza. If facing the fort entrance, the archway on the right leads to the Austria Battery, and from there you can directly access the curtain terreplein. If you take the archway on the left, you will first be on the Carmen Bastion. From there, take the ramp up to the Ochoa Bastion to access the curtain terreplein.

View towards the curtain terreplein from the Austria Bastion of Castillo San Felipe del Morro, San Juan National Historic Site

There have been very few modern modifications to the curtain terreplein since the 1770s. A water tank was installed above the sally port (fort entrance) by the United States Army in 1902, but it was removed in the 1920s. A new terrazzo floor was installed between 1938 and 1940 as part of a fort restoration by the Works Progress Administration, an organization created to get men back to work during the Great Depression. The WPA was noted for converting run-down public lands near urban areas into parks for inner city residents to enjoy. It was disbanded in 1943, as all the men who were put to work renovating urban areas were put to work fighting Germany and Japan.

In contrast to the curtain terreplein, there were many alterations done to the terreplein of the Great Wall. In anticipation of a war with the United States, Spain installed three Ordóñez guns in 1897, one at each corner and one in the center. A gunpowder magazine was built next to each gun. The center emplacement has since been covered by a World War II-era visual signal station installed by the United States Army. The Army also built a fire control tower on the northern corner (end near the lighthouse). This was used to spot enemy ships and to help provide firing coordinates for artillery. During the war there would have been some sort of scope inside, and those on duty would mainly have been looking for German U-boats.

Modern modifications to the Great Wall terreplein

Regardless of where you are on the Cavalier Battery, you will have good views of the Plaza de Armas. In the photo below, notice the triangular building at the top of the Great Wall on the left—a gunpowder magazine—and the structure with the antennae towards the center of the photo—top of the fire control tower. Another gunpowder magazine is in front of the tower.

View of the Plaza de Armas from the Cavalier Battery of Castillo San Felipe del Morro, San Juan National Historic Site

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