As the National Park Service celebrates its 100th anniversary this week, St. Augustine’s Castillo de San Marcos is being recognized for its place among America’s most important historic buildings. The Castillo is the oldest city’s most enduring and iconic structure. It’s been the dominant architectural feature of St. Augustine for more than 300 of the city’s 451 years of existence. The Castillo was declared a national monument in 1924 and taken over by the NPS in 1933.