The demilune (the triangular construction opposite the fort’s sally port), incorporates an uncommon example of the “Plan of 1870,” which was a response to the failure of masonry fortifications and other lessons learned during the Civil War. Ironically, Gillmore, the army engineer responsible for the demise of Fort Pulaski, was also responsible for this upgrade c. 1874. The Plan of 1870 called for earthwork fortifications with underground magazines and shelters (bombproofs). Passageways leading to the gun batteries include a dogleg to block incoming artillery. The batteries were limited to two guns each and feature high earthen traverses to prevent flanking fire from dismounting the guns. Despite developments in rifling, the guns employed at this type of fortification were large smoothbore cannon, although sometimes these were rifled by inserting a steel sleeve in the bore. The Plan of 1870 was only partially implemented before the U.S. Congress abandoned funding for the program in 1876 as rapid developments in artillery outpaced defensive engineering. No new fortifications would be constructed until the 1890s, under a completely different program.