La Fortaleza was San Juan’s first Spanish fortification, built in the 1530s before construction began on the larger San Felipe del Morro. Like El Morro, La Fortaleza is both a UNESCO World Heritage Site and a U.S. National Historic Site. For more than 150 years, it has also served as the governor’s mansion. Free guided tours are offered on weekdays. Look for the mahogany clock that stands in a corridor: The outgoing Spanish governor struck it with a sword on the day the United States took control of Puerto Rico—forever marking the time Spain lost the colony.