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Fort Independence is a granite bastion fort that provided harbor defenses for Boston, Massachusetts. Located on Castle Island, Fort Independence is one of the oldest continuously fortified sites of English origin in the United States. The first primitive fortification, called "The Castle", was placed on the site in 1634 and, after two re-buildings, replaced circa 1692 with a more substantial structure known as Castle William.

Castle Island is a peninsula in South Boston on the shore of Boston Harbor. In 1928, Castle Island was connected to the mainland by a narrow strip of land and is thus no longer an island. It has been the site of a fortification since 1634, and is currently a 22-acre (8.9 ha) recreation site and the location of Fort Independence.