The urgencies of the Civil War required that fortifications be constructed rapidly and at minimal cost. Forts begun in the Third System were completed, but new construction was primarily of wood-revetted earthworks. Sometimes these earthworks were constructed near a Third System fort supplementing the firepower of the fort and sometimes they were stand-alone fortifications. Often, they were constructed to close passageways that had increased in importance during the war. They were, due to the expediency of war, temporary in nature. In addition, the war saw the first use of underwater mines as a planned part of seacoast defenses.
Following the war, construction began on several Third System
forts in New England. Built of stone, and designed to accommodate
the large-bore cannon developed during the war, these massive
forts would have been formidable works. In 1867, however, money
for masonry fortifications was cut off and the Third System came
to a close. A combination of controversy over the vulnerability
of masonry to rifled cannon and large-caliber smoothbore cannon
and a lessening of concerns regarding land-based attack led to
the construction of masonry-revetted earthen fortifications. During
the 1870s, a number of these new works were begun which were to
include large caliber mortars and submarine mines. While the
cannon emplacements with their brick-lined magazines were constructed
and armed, facilities for the mortars and mines were not completed.
Most of the defenses were abandoned by the early 1880s.
