Castillo de San Marcos in St. Augustine, FL

It took 23 years to build. Those crazy Spaniards! They barged all of the stone used in construction from Anastasia Island, across the Matanzas River. Of course they let the Native American Indians and Cubans do all of the heavy lifting. Completed in 1695, the fort was never taken by force in its 251 years as a military stronghold, although it changed hands via treaty several times. Since 1933, the 20+ acre site has been operated by the National Park Service, and is the preeminent tourist destination in St Augustine today.