Like most other masonry forts built in the United States during the middle 1800s, Fort Popham is made up of a series of casemates. These are large enclosed spaces with high, arched ceilings and places for cannons to fire through wall openings. All of the nation's major masonry forts built from 1816 to 1867 had at least one level of casements.
Casemates were first developed in European forts centuries ago. By enclosing cannons within thick walls, casemates protected cannons and soldiers firing them. But firing large cannons within confined spaces also created safety and structural problems.
In addition to providing protection for cannons and soldiers, this open style of casemate used in Fort Popham allowed it to have several tiers, thus concentrating the firepower of a single fort.
Though this fort is built as a two-tier casement system, some forts of the period has three and four tiers of casements and cannons.
The casemates are open in back and have vents in the ceilings in order to disperse the smoke and noise from firing the cannon. The casemates were not joined structurally to the fort's front wall, so that they would remain intact even if the wall was destroyed.
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Construction of each casement began with a wood framing that supported the arched ceiling while it was being built. Once the brick and stone were laid in placed [sic] with mortar to hold them together, the arch shape maintained pressure that made the structure strong and stable. Once the casement was finished, the wooden supports were removed and used to build the next casement. This design is so strong that one can be built right on top of another.