Coquina walls of the Castillo de San Marcos rise ahead of you. This is the oldest masonry fort in the continental United States, anchored on the western shore of Matanzas Bay in St. Augustine, Florida. It was designed by the Spanish engineer Ignacio Daza, and construction began in 1672 — 107 years after the city was founded by Spanish Admiral and conquistador Pedro Menéndez de Avilés, when Florida was part of the Spanish Empire.
The decision to build this enduring stone fort followed a destructive raid by the English privateer Robert Searles in 1668, an event that led Governor Francisco de la Guerra y de la Vega to order a replacement for the damaged wooden fort. Work continued under Manuel de Cendoya in 1671, and the first coquina stones were laid in 1672. The core of the current fortress was completed in 1695, though it would undergo many alterations and renovations over the centuries.
When Britain gained control of Florida in 1763 pursuant to the Treaty of Paris, St. Augustine became the capital of British East Florida and the fort was renamed Fort St. Mark until the Peace of Paris in 1783 returned Florida to Spain. These walls of coquina outlasted Spanish rule, British rule, and the return of Spain in 1783.