Over the next 20 years the island was dramatically altered, and one of the country's finest forts was built. Dedicated in 1847, Fort Warren's defensive design was virtually obsolete upon completion. However, the fort served as a training ground, patrol point, and Civil War prison that gained a favorable reputation for the humane treatment of its Confederate prisoners. After 100 years of military use, the fort was decommissioned in 1947 and acquired by the Metropolitan District Commission for historic preservation and recreation in 1958.