The Castillo de San Marcos was a typical example of European design transplanted to the Western Hemisphere. It was a style of fortification evolved from the medieval castle. There was no great change in siegecraft and fortification until the gunpowder cannon came into use, but when that weapon did make its appearance the military engineers found themselves in a predicament. The towering walls of the ancient castles were conspicuous targets for the skilled artillerist. Adamant stone walls that had splintered the powerful crossbow shaft and resisted for days on end the pounding of the catapults tumbled into rubble after a roaring bombardment from heavy siege cannons. So the engineers lowered their targetlike walls, and in front of them they piled thick and high hills of earth to stop the cannonballs before they could hit the stone. Yet, because those walls still had to be too high for the scaling ladders, the surrounding moat was retained. Circular towers common to the older castles eventually gave way to the more scientific bastion, an angular salient from which the pikemen, harquebusiers, and artillerists could see to defend every adjacent part of the fort walls. The ultimate result was a rather complicated series of straight walls and angles—a sort of defense-in-depth plan—and in the center of it could usually be found the garrison quarters and the magazines. Fortification was a remarkably exact science, and one that was universally respected. “Many ... arguments,” wrote an eighteenth-century expert, “might be alledged to prove the usefulness of fortified places, were it not that all the world is convinced of it at present, and therefore it would be needless to say any more about it.” A fort, however, can never win a victory. Primarily a defensive weapon, it protects vital points and delays the invader. It can also be, as was the case with the historic fort in Florida, a citadel and a pivot of maneuver for colonial troops. For most defense problems, there was an answer in the book, though the brilliance of the engineer might well be measured by his ingenious use of natural defenses, as was the case at Castillo de San Marcos. There were as many different kinds of forts as there were uses for them. They promoted and protected trade, they guarded the pass into a country, or, like San Marcos, they secured the country from invasion. The following dogma, written three-quarters of a century after the castillo was started, might have referred specifically to the fort at St. Augustine: “In small states ... which cannot afford the expense of building many fortresses, and are not able to provide them when built with sufficient garrisons and other necessaries for their defence, or those whose chief dependance consists in the protection of their allies; the best way is to fortify their capital, which being made spacious, may serve as a retreat to the inhabitants in time of danger, with their wealth and cattle, till the succours of their allies arrive.”