El Morro Fortress in San Juan, Puerto Rico

Its weathered appearance gives it an almost ancient feel, but El Morro Fortress last saw military use as recently as 1961. The fortified lookout at the edge of San Juan does have some centuries under its stony belt, dating to 1539 when Puerto Rico was still held by Spain (itself ruled by Holy Roman Emperor Charles V). Despite ensuing political twists and turns back in Europe, Spain kept its hold on Puerto Rico until the Spanish-American War in 1898, after which this highly strategic gateway to the Caribbean became a territory of the United States. Part of the National Park Service since the US Army retired it as a base, El Morro Fortress was also designated a UNESCO site in 1983.