Fort Popham, like most masonry forts built in the United States during the middle 1800s, consists of a series of casemates, large enclosed spaces with high arched ceilings and wall openings for cannons. All of the nation’s major masonry forts built from 1816 to 1867 had at least one level of casemates, a design first developed in European forts centuries earlier. By enclosing cannons within thick walls, casemates protected both the guns and the soldiers firing them, though firing large cannons in confined spaces also created safety and structural problems. The open style of casemate used at Fort Popham provided protection while allowing several tiers, concentrating the firepower of a single fort; Fort Popham was built as a two-tier casement system, while some forts of the period had three and four tiers of casemates and cannons. The casemates are open in back and have vents in the ceilings to disperse smoke and noise from cannon fire, and they were not joined structurally to the fort’s front wall so they would remain intact even if the wall was destroyed. Each casemate was built with wooden framing to support the arched ceiling during construction, then brick and stone were laid in place with mortar, and the arch’s shape created pressure that made the structure strong and stable. After a casemate was finished, the wooden supports were removed and reused to build the next one, and the design was strong enough for one casemate to be built directly on top of another.