The Spanish built and manned Fort Matanzas (1740-42) to ward off British attacks on St. Augustine.
Visitors need to understand that the fort is located 14 miles south of St. Augustine (along A1A). The area, now Fort Matanzas National Monument,  is run by the National Park Service and located on Anastasia Island. The park is situated near the site of the killing of nearly 250 French Huguenots in 1565 by the Spanish, an act that gave the river and inlet the name Matanzas, Spanish for “slaughters.”
Upon arrival (free parking) watch the eight-minute film to learn about the fort and the area’s history. Then, take a Park Service boat over to Rattlesnake Island, a less than 5-minute boat ride. Rattlesnake Island, a barrier island is left to wildlife, except for official trips by the Park Rangers. The public may boat and fish the waterway, but are not permitted to use the fort’s dock.

Fort Matanzas measures only 50 feet on each side with a 30-foot tower; so a visit becomes a quick exploration. If possible go on a day when the soldiers are in costume.

Here is a soldier near the Garita or sentry box.

A Spanish flag flies from the observation deck. You’ll also find a chimney for the hearth below.The powder magazine was build into the land-based side of the fort’s walls.

A cistern for water storage lies below the canon deck, but is not open to tourists.

When the soldiers fire the canon, all visitors must evacuate the structure.  Park Rangers gather them outside, and then explain the procedure and answers questions. The location allows only a side view of the canon from below, so you can’t see much of the soldiers’ participation in the activity.  As one of the reenactors said, “If you really want to watch a canon firing, go to the big fort- Castillo de San Marcos.”

History
The Spanish landed in St. Augustine in 1565, claimed it and built a settlement.  Francis Drake raided the town in 1586. Afterward,  the Spanish erected Castillo de San Marcos for their protection, a massive coquina fort still standing in the city (completed in 1695). In 1740, Governor James Oglethorpe and his British troops from Georgia blockaded the St. Augustine inlet or harbor. The Spanish held Castillo de San Marco during the 39-day siege, which was halted when hurricane season arrived and Oglethorpe withdrew.
To prevent the British from attacking via the Matanzas River (a weak point in the city’s defense at the  rear) the Spaniards constructed an outpost –Fort Matanzas. Oglethorpe returned in 1742 with 12 ships, but the soldiers drove off the attack with the little fort’s canon. Fort Matanzas was never attacked again.

Like Castillo de San Marco, Fort Matanzas was built of coquina stone and covered inside and out with white lime plaster. Usually, only one officer, four privates of the infantry and two gunners manned the fort. Soldiers were assigned there as a part of their regular rotation among the outposts and missions near St. Augustine. The tour of duty at Fort Matanzas was one month.

What happened to the fort?
As part of the Treaty of Paris in 1763, signed to end the French and Indian War, all property in Florida was transferred to Britain. After the American Revolution, a second Treaty of Paris returned Florida to Spain. Fort Matanzas continued to be staffed but was not maintained. When Florida became a state in 1819, Spain transferred the land to the US. The fort had become so badly deteriorated that soldiers could no longer live inside. All that remained were two eight-pounder Spanish cannons originally mounted in 1793. They remain to this day. The US took possession in 1821 but never occupied the site.
Military personnel were later sent to examine the ruins. They determined that Fort Matanzas had only historical value as the exterior surfaces were overrun with vegetation and its walls had cracked.

History lovers gained Fort Matanzas on July 18, 1916, when $1025 was granted by Congress for the repair of the historical structure. On October 15, 1924, using the power granted in the Antiquities Act, President Calvin Coolidge named five sites, including Fort Matanzas and the Castillo de San Marcos, as national monuments. On August 19, 1927, he issued another order, assigning all the lands around the fort, not included in the national monument to the Department of Agriculture, as a bird refuge.