In St. Augustine, Florida, Castillo de San Marcos National Monument is the oldest fort of masonry construction, initially built by Spaniards in the 17th Century. The unique stone used for this fort was shipped in from an offshore island in the Atlantic Ocean. The composite rock was largely compressed sea shells, making it very durable and able to withstand quite a pounding from enemy fire.
During the colonial wars that took place for over 300 years, this fort weathered many attacks and battles, primarily with British armies. Over the course of three centuries, the fort traded hands between Spanish and British forces several times. Ultimately Spain signed over the entire Florida territory to the United States, ending the years of international fighting for control over this important military stronghold. The name was then changed to Fort Marion.
The fort later came into play during the US Civil War. Originally occupied by Union soldiers when Florida seceded from the United States in 1861, those soldiers all left with the exception of one “caretaker” who stayed behind. When Confederate troops arrived later that year, he relinquished control of the fort. In early 1863 the Union army retook control of Ft. Marion.
The fort was used throughout the US occupation as a military prison, largely housing those who deserted the Spanish-American War and displaced Native Americans from the American westward expansion. In 1900 it was closed as a military installation and subsequently designated a National Monument in 1924.
There is a $5 admission fee to enter this US National Monument.