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La Fortaleza (English: The Fortress), also known as Palacio de Santa Catalina (Saint Catherine's Palace), has been the official residence of the governor of Puerto Rico since the 16th century, making it the oldest executive mansion in continuous use in the New World. Built between 1533 and 1540 by orders of Charles I of Spain, the structure was the first fortification constructed by the Spanish on San Juan Islet to defend San Juan Bay, the harbor of Old San Juan. La fortaleza, alongside El Morro, San Cristóbal, El Cañuelo, and other forts part of the Walls of Old San Juan, protected strategically and militarily important Old San Juan, La Llave de las Indias (The Key to the Indies), from invasion by competing world powers and harassment by privateers and pirates during the Age of Sail. It was designated a World Heritage Site by UNESCO in 1983 as part of La Fortaleza and San Juan National Historic Site.