St. Augustine, Florida, is the oldest European settlement and the only walled city in the United States; its guardian, Castillo de San Marcos, is the oldest standing fortification. Throughout most of the sixteenth and seventeenth centuries, though, few would have given a clipped doubloon for its survival. In fact, the viceroyalty of New Spain (Mexico) would gladly have ignored its existence. Mexico had to foot the bills for the upkeep of the lonely outpost, which couldn’t even feed itself.

In the early 1560s, when Adm. Pedro Menéndez de Avilés organized his system for convoying galleons loaded with New World treasure back to Spain, France planted at the mouth of the St. Johns River a colony from which privateers could sally to harass the homebound fleets as they beat up past the Bahamas.

Menéndez reacted ruthlessly. In 1565 he smashed the little French colony and ordered the massacre of some two hundred and fifty helpless prisoners. Then he established St. Augustine and Fort San Marcos to ensure it wouldn’t happen again.

Militarily the site was excellent. Town and fort were located on a narrow, sandy peninsula with almost impassable marshes on three sides and the fourth side squeezed by a meandering stream. Fort San Marcos commanded the entrance to a long, narrow bay. A shallow sandbar across the harbor mouth kept out large enemy warships.

Before long coquina stone was discovered on nearby Anastasia Island. Coquina stone, a soft stone made of broken seashells cemented together in their own lime under tremendous geological pressure, is ideal for building fortifications. Menéndez immediately recommended a stone fort at St. Augustine, but nobody in Madrid or Mexico City would buy the idea.

For nearly a hundred years the scraggly settlement lived from hand to mouth while nine wooden stockade forts fell victim, one after another, to time, fire, weather, and termites. Since nothing happened, faraway authorities were lulled into a false sense of security. Every suggestion for bringing San Marcos up to scratch was talked to death.

Then, in 1668, a privateer crew surprised and captured the town. The attackers were English, and they made it clear they meant to come back. The threat was driven home two years later, when England founded Charleston, South Carolina, on land claimed by Spain.

The Spanish government ordered an immediate reconstruction in stone of San Marcos, increased the garrison, and told the viceroy of Mexico to pay up. Construction finally got under way in 1672, but after a fast start the project slowed down, and the new Castillo de San Marcos wasn’t finished until 1695.

It was a square structure with triangular bastions at each corner and a ravelin covering the south curtain, which contained the only entrance via a drawbridge over the ditch. Walls of coquina mortared together with oystershell lime cement rose thirty-three feet from a twelve-foot-thick base. Because the wooden firing deck wasn’t strong enough to withstand the shock of recoil,