Historic Marker number 98 is on Fort Road between Petronia and Olivia Streets.
This historic site contains the last remaining Endicott Period coastal harbor defense battery in Key West. Endicott Period fortifications were constructed from 1890 to 1910. There were originally four concrete encased batteries on the harbor side of Fort Zachery Taylor. One was located in what is now Truman Annex. Another was at the harbor end of Southard Street and another was located at Historic Marker # 54 in the Fort West Martello Tower.
The batteries were not fortresses, but a system of well-dispersed emplacements with a few large guns in each location. The structures were usually open-topped concrete pits protected by sloped earthworks.
The Battery Seminole has two concrete enforced mortar pits separated by a central magazine that served both pits. Each of the pits housed two 10" mortars and mortar carriages. The site no longer has any armaments in the pits. Some of the mortars were removed and sent to Europe to support war efforts during World War I and II. The site was in active service from 1904 to 1942.
The two concrete steps leading to platforms on either side of the mortar pits at Battery Seminole were designed to triangulate gunfire. Triangulation is used to calculate the coordinates and distance from the shore to a desired target. With the coordinates of each watch tower known, a mathematical equation known as the Law of Shines was used to direct the accuracy of mortar fire.