In 1811, the town of Fernandina was platted according to the Spanish Law of the Indies, which prescribed how the site for a Spanish settlement should be selected, and how the town should be laid out in classical grid form. This law also stated that there must be a means of fortification to protect the town and a centrally located plaza for use by all residents. Fort San Carlos was built to fulfill this need.
The Spanish built the wood and earth fort in 1816 during Spain's second rule over Florida. Containing eight to 10 guns, the fort was occupied several times by invading forces, including the United States in 1817. The fort was soon abandoned after the 1821 transfer of Florida from Spain to the U.S..
Fernandina Plaza Historic State Park is the site of the historic plaza grounds that once laid in front of the Spanish Fort San Carlos. State archaeologists estimate that at least two-thirds of Fort San Carlos has been lost to erosion and the plaza area is all that remains.