The exploration and colonization of Florida by Europeans was a maritime undertaking. Because people and goods moved by ships at sea and on rivers, safe inlets and protected harbors were most important factors in deciding where to settle and what to defend. From a secure seaport, settlement could extend into the unknown and challenging interior, but constructing and maintaining adequate roads, and transporting goods by animals and carts was exceptionally difficult.

Florida was under Spanish control, to varying degrees and with one 20-year British exception, for almost 300 years. From 1565 St. Augustine was the administrative center, situated on the “Highway of the Indies,” now known as the Gulf Stream, that propelled sailing fleets from the New World back to Spanish ports. Here was the formidable Castillo de San Marcos, and the smaller forts Matanzas to the south and Mosé to the north, that controlled entrance to the inlet, the harbor and the connecting rivers. The Castillo is the oldest masonry fort and the only 17th century fortification in the nation. On the Gulf Coast, Pensacola was the focus of Spanish settlement beginning with the failed attempt at settlement by Tristán de Luna in 1559. A series of Spanish forts guarded the entrance to Pensacola Bay including Fort San Carlos de Austria (1698-1719) and Fort San Carlos de Barrancas (1787-1814, rebuilt 1817).

On opposite banks of the St. Johns River, Fort San Francisco de Pupo and Fort Picolata were constructed in the early 1700s to guard the river crossing of the Camino Real and defend the river south of St. Augustine. Some missions also had their own fortifications during the late 17th century, including at Mission San Luis de Apalachee in present-day Tallahassee. South of Tallahassee on the Gulf coast Fort San Marcos de Apalache was established in 1679 to defend the St. Marks and Wakulla River entrances to the inland Apalachee region.