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.
Is Castle Island really an Island?
No, Castle Island is a peninsula off Boston Harbor as of 1928, when it was connected to the Massachusetts mainland.  It is a recreation area that is the site of Fort Independence.