Fort Independence on Castle Island (island-to-peninsula change)

Castle Island was historically a separate island in Boston Harbor but is now connected to South Boston by a causeway and shoreline alterations; it is the site of Fort Independence, a granite fortification whose current structure dates largely to the 1830s. The causeway and adjacent fill have changed access and the island's relationship to the mainland, so the fort sits on what is known as Castle Island but is functionally a peninsula. Fort Independence and the park preserve a visible instance of Boston's modified harbor geography.