El Castillo de San Felipe del Morro

More commonly known as El Morro, this stone garrison is the second-oldest freestanding fort in the Americas. Built between 1539 and 1797 by Spanish settlers on a rock-strewn promontory at the entrance of the San Juan Bay, it's a hulking pile of turrets, towers, walls, barracks, and dungeons that are wonderful to explore. A combination pass buys entry to both El Morro and Castillo San Cristóbal, another fort on the northeast edge of Old San Juan; it's larger than El Morro and usually less crowded.