Cannon on the main terreplein of Castillo San Cristóbal, San Juan National Historic Site

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Castillo San Cristóbal Main Page

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A terreplein is a level area of a fort where artillery is positioned. A fort may have various terrepleins, but there is typically one location where most of the firepower is concentrated. Park signage at Castillo San Cristóbal refers to this level of the fort as the Main Firing Battery. To access it from the plaza de armas (main plaza), take the center of three tunnels on the north end of the plaza—can’t miss them—or the tunnel on the right just as you start up the ramp to the caballero (cavalier), the tallest structure in the fort.

Tunnel to the main terreplein of Castillo San Cristóbal, San Juan National Historic Site

Second tunnel to the main terreplein of Castillo San Cristóbal, San Juan National Historic Site

To the best of my knowledge, the main terreplein was part of the original Castillo San Cristóbal from the mid-1600s. When the fort was renovated between 1766 and 1785, creating what largely stands today, the terreplein on the north bastion (the protruding section at the north end of the fort near the ocean) was raised six feet while the terrepleins of the south bastion and the curtain (wall between two bastions) were raised three feet. Since the entire terreplein is now the same height, I assume the terrepleins were not level when originally built.

Castillo San Cristóbal

Constructing new terrepleins also requires a new parapet, which is the wall soldiers stand behind for protection when shooting guns or artillery at the enemy. And of course there has to be embrasures, which are holes or gaps in the parapet to shoot out from—gaps in this case. When construction finished in the late 1700s, there were 23 embrasures on the east wall and 14 on the north wall facing the ocean. Today there are 22 on the east wall. There is a chunk of the parapet missing on the north bastion, creating a single gap. The same spot on the south bastion has two embrasures. This would account for the missing embrasure, but what happened to the north bastion parapet is anyone’s guess.

Cannon shoots through an embrasure in the parapet on the terreplein of Castillo San Cristóbal, San Juan National Historic Site

There is also a missing embrasure on the north wall, but this was removed when the United States Army built a fire control station on this spot during World War II. Fire control stations were 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. The fire control station is open, so you can check out the inside.

Entrance to a World War II fire control station on the main terreplein of Castillo San Cristóbal, San Juan National Historic Site

View from inside the World War II fire control station on the main terreplein of Castillo San Cristóbal, San Juan National Historic Site

Castillo San Cristóbal always had its own version of a fire control station, the sentry box. There is one on the north bastion that is open (note that the photo below is of a typical sentry box but not the one on the north bastion). When inside, look down to the ocean shore on your left and you will see another sentry box, Fuerte del Espigón (Fort of the Point, aka Devil’s Sentry Box). This was part of the original fort. It was restored by the National Park Service in 2005 and is now open to the public. Visiting it, however, does require a short hike through extremely overgrown terrain—at least when I visited; perhaps Rangers will have weed-whacked the trail by the time you visit. See the Hike to Devil’s Sentry Box artricle here on National Park Planner for details on how to get to it. If you are a photographer or just want some great photos, this is the place to be.

Typical sentry box on the Spanish forts at San Juan National Historic Site

Devil’s Sentry Box, part of the original 1630s Castillo San Cristóbal, San Juan National Historic Site

The only other modifications to the terreplein are two gun emplacements for modern artillery that were installed by the Spanish in anticipation of war with the United States in the late 1890s. A protective wall that stretches between the two was also built. One of the emplacements is next to the fire control station (see photo above). War did break out in April 1898—the Spanish-American War. It was over that December, with the result being an American victory. Per the terms of the Treaty of Paris, the United States took possession of Puerto Rico, the Philippines, and Guam.

Emplacement for modern artillery added to the terreplein of Castillo San Cristóbal by the Spanish around 1897 in anticipation of war against the United States

Protective wall around the new gun emplacements added to the terreplein of Castillo San Cristóbal by the Spanish around 1897 in anticipation of a war against the United States

The main terreplein is a great place to come for views of the outworks of Castillo San Cristóbal, all of which were built in the late 1700s. These include the V-shaped San Carlos Ravelin directly in front of the fort, another V-shaped fortification at the far end of the compound called the Fort of the Fan, and a few other artillery batteries. The grounds and structures have been closed to the public since 2019 for renovation. A Ranger told me that she didn’t know when, or even if, the grounds will open again.

View of the Castillo San Cristóbal outworks and grounds from the main terreplein, San Juan National Historic Site

You can also get fantastic views of San Juan from the main terreplein.

View of San Juan from the main terreplein of Castillo San Cristóbal, San Juan National Historic Site

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