Fort Independence (eighth) is built on Castle Island.

Architect: Thayer, Sylvanus. Built of Rockport granite, the five-bastioned fort is doubled in height and expanded in its perimeter. During the civil War, it is used as a training site for Union soldiers and a prison for deserters. The fort is de-commissioned by the federal government in 1879, re-commissioned in 1898, and used as an observation and defense station in World War I. During World War II, it is used as the site of a facility that de-magnetizes ship hulls to protect them from German mines. The fort is deactivated in 1947, and the site is purchased by the Metropolitan District Commission in 1962.