In historic unreinforced masonry structures, one of the most critical structural issues is the differential settlement of foundations. Due to unreinforced masonry's brittle nature with a fairly low tensile strength, brittle cracking can occur due to tensile stresses introduced by foundation settlements. This study demonstrates the development and calibration of a finite element model and the use of this model for structural analysis under differential settlements of a casemate of Fort Sumter, a masonry coastal fortification best known as the site where the first shots of The American Civil War were fired in 1861. Development of accurate finite element models for historic masonry structures presents numerous challenges in the acquisition of non-linear material properties, and irregular geometry. Furthermore, these challenges are exacerbated because of the configuration of coastal fortifications, as these structures have characteristic designs unique to the distinct functionality of defense, such as cold-joints between disjointed structural components. The non-linear material behavior of the brick and mortar assembly is obtained from laboratory tests on samples obtained at the site. High definition laser scanning is used on irregular geometry to obtain the details of accumulated structural damage and degradation, including differential foundation settlements. The uncertain interface behavior at the cold joint between the scarp wall and the casemate of the fort is assessed using in-situ vibration tests. The finite element model developed is utilized to study the settlement magnitudes critical to the stability of the casemates of Fort Sumter for a variety of possible soil settlement configurations.