Park rangers at Fort Independence at Castle Island in South Boston (©Massachusetts Office of Travel and Tourism)
Five-bastion Fort Independence has been the site of Boston’s sea defense since 1634, although the existing structure dates to 1851. The pentagonal fort was primarily used for training local soldiers and confining prisoners of war as early as the American Revolution. Today, you can go inside the National Historic Site on free guided tours, or explore the grounds on your own. Tours May 24-Oct.13 (meet at entrance to fort): Sa-Su noon-3:30 pm. Twilight Skyline Viewing Th 7 pm-dusk.