Is Castle Island really an Island?

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.[3] It has been the site of a fortification since 1634,[3] and is currently a 22-acre (8.9 ha) recreation site and the location of Fort Independence.[2]

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.