El Morro
With a history that dates 500 years, this stone structure truly deserves its place of honor in the National Historic Register. The English and the Dutch attacked the Morro repeatedly in its time. Located on the San Juan Bay, the six-story fort was originally constructed in 1540, although its exterior walls were not erected for another 200 years. Inside the walls is a veritable labyrinth of chambers, barracks, hallways and prison cells. The fort was given a complete facelift in 1992.