The beautifully preserved Fort Knox in Penobscot, Maine, was built with granite in the mid-19th century to guard the Penobscot River from a potential British invasion. The attack never came. The fortress was also manned during the Civil and Spanish-American Wars but still never saw a military conflict. The 124 acre, former military post is now the Fort Knox State Park. It is fun walking around this U. S. National Historic Landmark.