The Castillo de San Marcos site is the oldest masonry fort in the United States. It is located in the city of St. Augustine, Florida. Construction was begun in 1672 by the Spanish when Florida was a Spanish territory.

Decius:

This is the closest thing to a medieval castle that I've seen in North America. It's a really beautiful building.

The US National Park Service also runs Fort San Felipe del Morro in San Juan, PR. "El Morro" was begun in 1539 as a Spanish fort; it became a US fort at the end of the Spanish-American War. It is modeled in Google Earth, which offers a perspective not captured in most visitors' snapshots.

RE: Castillo de San Marcos - Wikipedia, the free encyclopedia