The Castillo remained a vital outpost, but Florida increasingly became a contested zone between Spanish authority, American expansion, and Native resistance. Enslaved people seeking freedom often fled to Florida, where the Spanish occasionally offered sanctuary, while Seminole groups formed alliances and defended their territories.
By the early nineteenth century, the United States viewed Florida as both a security risk and an opportunity for expansion. The Castillo, once a bulwark of empire, now stood as a reminder of Spain’s waning influence.
American Acquisition and Fort Marion
In 1821, Spain formally ceded Florida to the United States under the Adams-Onís Treaty. The Castillo was renamed Fort Marion, in honor of Revolutionary War hero Francis Marion, the “Swamp Fox.”
As an American installation, Fort Marion served various roles beyond traditional defense. During the Second Seminole War (1835–1842), the fort was used to detain Seminole prisoners, including the famous leader Osceola, who was captured under a flag of truce and later died in custody.
In the latter half of the nineteenth century, Fort Marion became a military prison. During the 1870s, it held Native Americans from the Great Plains, including Apache and Cheyenne leaders, who were transported far from their homelands. Their imprisonment at Fort Marion reflected the broader U.S. policy of forced relocation and assimilation of Native peoples shutdown123