Part of Fort Jefferson's modernization program of the 1870's was the construction of eighteen traverses along the terreplein, or barbette tier. Each of these batteries included two Rodman Guns on pintle mounts, with a traverse magazine betwixt. This concept came about when everybody saw what happened to such American forts as Fort Sumter, Fort Macon and Fort Pulaski during the Civil War: They were pummelled to defeat in short order by rifled cannon, proving the masonry fort to be considerably less impregnable than previously imagined. It was thought that guns atop forts, as opposed to being in those fragile casemates, would be more survivable in the future. Set aside the fact that they're still supported by all those delicate casemates. A nearby sign relating to the traverses: Traverse Magazine.