Fort at the Edge of the World: Castillo de San Marcos National Monument, April 2008 Today was the last day for our little excursion. We decided to stop in St. Augustine for a bit on our way down the coast. St. Augustine is the oldest continuously occupied European-established city, and the oldest port in the continental United States. When construction began on the Castillo de San Marcos in 1672, this masonry fort was literally on the edge of the known world to the Europeans who settled here.