Construction of Fort Jefferson was begun on Garden Key in December 1846, under the supervision of 2nd Lt. Horatio Wright, after plans drawn up by Lt. Montgomery C. Meigs were approved in November. Meigs’ plans were based on a design by Joseph Totten.
The design called for a two-tiered casemates in a six-sided outline, with two curtain walls measuring 99 meter, and the other four measuring 145 meter. Corner bastions, which are large projections designed to allow defensive fire along the faces of the walls they joined, contained gun-rooms, gunpowder magazines and a granite spiral staircase. Each tier of casemates contained 150 guns, and another 150 were placed on top of the fort itself.
The heavy guns were mounted inside the walls in a string of open casemates, or gun-rooms, facing outward toward the sea through large openings called embrasures. The 13-acre parade ground contained additional powder magazines, headquarters, a hospital, officer quarters and three large barracks.