Fort Taylor is located on Key West, the southernmost of the inhabited Florida Keys. It was originally constructed as a multi-tier Third System fort, beginning in about 1845. Construction was slow due to disease, storms, the Civil War, and a generally challenging climate, but it was generally complete by shortly after the Civil War. Fort Taylor was Union held throughout the war.
An image of Fort Taylor from about 1895. You can see it was a very imposing fortification of multiple tiers, completely surrounded by water.
Another image from the same time frame, showing the land face of Fort Taylor, and the causeway leading to the fort.
During the late 1890's and early 1900's, defenses were upgraded with the addition of Endicott period concrete gun batteries. To accommodate them, the upper tiers of Fort Taylor were removed, many of the casemates were filled with sand or concrete and sealed, and concrete was used to stabilize much of the brickwork.