Fortifications and Siege Craft

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Fortifications and Siege Craft


Types. By 1350 medieval fortification design had reached its apex. Little change would take place for more than a century, until the development of gunpowder artillery began to have an impact. Medieval fortification had three major functions. The most-common one was serving as the fortified residence of a powerful lord, which is the proper meaning of the word castle. Fortifications were used also to defend towns and cities. The third type of fortification, royal forts with small garrisons to defend the frontiers of a realm, was fairly uncommon. There often was overlap between the basic types, such as the well-fortified city of Carcassonne in southern France, which served to block the key invasion route from Spain, or the royal forts built during the English occupation of Wales in the thirteenth century, which became the castles of nobles in the following century.

Castles. The typical castle was the residence of a baron, whose place in the feudal system ensured him of enough wealth to build a small castle and garrison it. It had stone walls thirty to forty feet high surrounded by a ditch. There were several round towers, usually at the four corners of a small square castle, that projected both above the wall and in front of it to provide flanking fire. Both the wall and the towers had long, narrow windows, small enough that a man could not crawl through, but large enough to allow bowmen to fire at attackers. At the top of the wall and the towers was the checkerboard pattern of crenellations and merlons that provided both protection for the defenders and spaces for shooting at besiegers. Along the outside top edge was a projecting galley built into the stone of the wall with holes in its floor. Called a machicolation, its function was to allow the defenders to shoot or drop objects straight down to the base of the wall at any enemy sappers without exposing themselves by leaning over the top of the wall. A set of towers called a barbicon defended the gate of the castle, without which the gate was the weakest point of the castle. The barbicon projected forward of the gate, creating a short tunnel in front of the gate. Holes in the ceiling of the tunnel allowed defenders to rain objects down on attackers who tried to batter down the doors. The doors were usually massive wooden structures reinforced with iron bars and covered with leather to prevent fire; they took several minutes to close and secure. To deter a surprise attack that might catch the doors open, the gate had an iron portcullis that could be dropped with the yank of a rope to temporarily shut off the entrance. If the castle had a moat around it, it had a counterweighted drawbridge designed to swing up rapidly to forestall a sneak attack. A large fortification with a substantial garrison had small postern doors hidden from the sight of those outside the castle designed to allow sallies against the attackers. Kings had large castles, such as the Tower of London or Vincennes outside of Paris, that served as their residences as well as strongholds to defend their capitals.

Sieges. A well-built stone castle outfitted with most of the above features and defended by a small garrison of reso-lute men was capable of withstanding a siege. Most mechanical siege machines did not throw objects of enough weight to do much damage to the masonry, nor were they accurate enough to hit the same spot twice and create a multiplying impact on the masonry to cause the wall to col-lapse and open up a breach. Mining was another option, which involved digging a tunnel under the wall. Once directly under the wall, the miners filled the tunnel with combustibles and set them on fire to burn away the supports of the tunnel’s roof and crack the masonry of the wall above. If all went well, the tunnel would collapse and with it part of the wall above. Mining, however, was slow and difficult and subject to the countermining efforts of the defenders, who often succeeded in forcing the miners to give up. Consequently, a passive siege intended to starve the defenders out was the most common method. This type of operation usually required months or even a year. However, the spread of disease and boredom among the besiegers, the cost of keeping a siege army in place for months at a time, and the possibility of the arrival of a relief army meant passive sieges were not successful against a well-stocked castle with strongly motivated defenders. It was this lack of success that allowed many feudal nobles to openly defy their kings during the Middle Ages.

Towns and Frontier Forts. The walls of towns and frontier forts were constructed little differently than castles. City walls, of course, could run for a mile or more, requiring towers every one hundred yards or so and barbicons for several gates. In Italy the towers tended to be even with the top of the wall and be broad in diameter. These squat-looking structures were called drum towers. The defenders of the urban walls consisted of the urban militiamen, recruited from among artisans and merchants who formed the ruling class of medieval cities. They did not trust the poor residents of their cities enough to put weapons in their hands, so they were forced to provide the defense of their cities themselves, despite the time it took away from their businesses. Because the urban militiamen lacked military training, they sought weapons they could use effectively from the walls with only a little practice, such as the crossbow and later the handgun when it became available. The major differences between a frontier fort and a castle were the absence of a relatively comfortable apartment for the lord of a castle and the presence of more of the spartan living quarters for the larger garrison used in a fort.

Advantages of the Besiegers. In most respects the defenders of a fortified place had the advantage over the attackers in 1350. Over the following century, the development of gunpowder artillery into effective siege weapons turned the advantage to the attackers. Defenders could use cannon atop the walls against attackers or through gun loops opened in the walls and towers and hookguns, which were handguns designed to hook onto the wall to absorb the recoil. They could not, however, overcome the defects of late-medieval fortification in the face of powerful cannon. While early cannon had problems of inaccuracy, slow reloading time, and a tendency to burst, they were still more effective than the traditional mechanical siege machines in breaching walls. Cannon had a somewhat better chance of hitting close to the same spot on the wall, improving the odds of crushing the masonry. More important, the flat trajectory of cannon shot meant that the balls would strike low on the high and relatively thin walls. When they did crush the masonry and cause the wall to collapse, the breach that was opened was close to the ground and easier for the besiegers to assault than a breach high on a wall. The rubble from a high wall collapsing was also more likely to fill in the moat, making it easier for the attackers to cross it. In the final two decades of the Hundred Years’ War (1337-1453), the French used cannon to take English-held towns and castles in rapid succession, playing a major role in the final ousting of the English from France. By the end of the war the French king had appointed the Bureau brothers as masters of the royal artillery train. They continued to develop better cannon and had a major role in creating the mobile gun carriage that allowed heavy guns to be transported at about the same speed as the rest of the army moved. Commanders no longer had to wait for days before the artillery would arrive for use in a siege or a battle. The major success of cannon in the fifteenth century was the siege of Constantinople (1453) by the Ottoman Turks. The Turks used huge bombards against the ancient but still powerful walls of the Byzantine city. Some of the Turkish bombards were so large that they were forged on the spot where they were fired, so that enormous effort would not have to be expended to move them into place. Constantinople, which had with-stood dozens of major sieges since its walls had been built in the fifth century, fell after a siege of one month.

Bronze Cannon. The crucial development in the late fifteenth century was the use of bronze for casting guns. High-quality bronze muzzle loaders called culverins were capable of using iron or lead balls rather than the stone balls used in iron cannon. Metal balls struck the wall with a great deal more force than stone ones and did not shatter upon impact as stone balls frequently did. Traditional fortifications were more vulnerable than ever before. With the demise of the medieval castle, nobles and kings began to change the style of their residences. Abandoning any hope of being able to combine defense and residence in one structure, they built luxurious palaces and chateaux that were pleasant places to live and to show off their wealth and culture. Since bronze was expensive compared to iron, only the kings could afford to put together a large artillery train of bronze culverins. This situation gave the kings a great advantage over their nobles, whose ability to defy the king by holding out in their castles was eliminated. It was a major step toward the European monarchs taking effective control over their kingdoms. Cannon played the major role in the victory of Ferdinand of Aragon and Isabella of Castile over the Moors, the final step in the Christian Reconquista of the Iberian Peninsula. The Spanish monarchs had an artillery train of eighty heavy guns that successfully brought the siege of the great fortress of Granada to an end in 1492.

Italian Trace. The excellent French artillery train played a major role during the first French invasion of Italy that began in 1494. King Charles VIII’s forces quickly took fortifications that the Italians expected would take them all summer to reduce. The conquered places were subjected to the traditional three days of sack, plunder, and rape that was the fate of the residents of fortified places that resisted, and their garrisons were executed. The ease with which the French took those places and the dire fate of the people within them persuaded the Italians to offer no further resistance, and Charles marched on south to Naples, his pri-mary goal. The Italians, however, responded to the humiliation of the easy French victories by searching for new ideas to improve fortification to withstand the onslaught of the French artillery. The best minds of the time, including Leonardo Da Vinci and Michelangelo, set to work to design better fortifications, but the most successful ideas came from lesser known men such as the Sangallos, a family of military architects, and Fra Giovanni Giocondo. Their solution to the problem of designing fortification capable of withstanding cannon became known as the Italian trace. They had to deal with the problem that high walls were vulnerable to cannon fire, yet to lower them increased the risk of scaling. Their solution dropped the wall down into a deep ditch usually filled with water, so that a cannonball could hit only the top of the wall, yet there was twenty to thirty feet of wall to make scaling diffi-cult. Crenellations and merlons were removed from the top of the wall, which was made rounded so that a cannonball was likely to bounce up and over it without striking at the full impact of a right angle hit. A space of up to one thou-sand feet was cleared of every building, tree, and bush to provide a clear field of fire for the defenders. It would often be laid with masonry to make tunneling up through it more difficult. This feature became known as the glacis. The bastion with gun openings, called casemates, was introduced; it projected farther forward of the wall than the medieval tower did in order to provide better flanking fire.

Bastion Improvements. Some early bastions were rounded on the assumption that cannonballs would glance off them. Albrecht Durer, a military engineer as well as an artist, designed forts with round bastions for Henry VIII on England’s south coast. Fortunately for the English, they were never tested by a siege, since the round bastions prob-ably would have been found vulnerable as those built in

Italy were. The principal problem was that the round bastion had a dead space in front of it that could not be covered from the main wall, now called the curtain wall, or from other bastions. The Sangallos solved the problem by using triangular bastions for the fort at Nettuno (near Rome) they began in 1509. It was a small square fort with a triangular bastion at each corner. It was designed so that guns at the flanks of a bastion could sweep every foot of the adjoining curtain wall and the face of the neighboring bastion. The cannon on the neighboring bastion did the same for it, and so on around the fort, leaving no spot on the exterior line of defense unprotected. Protruding shoulders protected the men and guns on the flanks of the bastions from hostile fire. Soon cities began to strengthen their walls by placing bastions at regular intervals on the long expanses of wall. As long as the architect did his geometry correctly, the bastions could provide covering fire for neighboring bastions all the way around the city. The Italian engineers also began to put masonry on the counter-scarp, the opposite side of the ditch from the wall, and cut casemates into it, so that anyone who came down into the ditch was caught in a murderous cross fire. A walkway was often placed a little less than a man’s height from the top of the counterscarp. Defenders used this covered way to fire at the attackers with firearms or small cannon.

Stalemate. As the Italian trace spread across Italy and into northern Europe after 1525, besiegers found themselves stymied by the new style of fortification. Sieges no longer could be successfully ended in a matter of days as in 1494. An Italian trace bristling with its own heavy guns made it highly dangerous to try to batter the walls from as close a range as had been true in the late Middle Ages. Nonetheless, the offense soon found a way to catch up to the defense. Trenches had to be dug from beyond the effective range of the defender’s heavy guns up the glacis to the top of the counterscarp. These trenches had to zigzag, so the defenders could not get a clear shot down them. A cannonball fired down a straight trench could roll for a long distance, killing or maiming a large number of men if they happened to be standing in it. The diggers were also protected by using gabions, cylindrical structures made of sticks and stakes bound together and filled with dirt, which were placed on the upper edges of the trench. Once the attackers’ trenches reached the top of the counterscarp, the attackers would drag their heavy gun up and fire point blank (without any elevation to the gun) into masonry of the opposite wall or bastions, using the gabions to protect the gunners and the guns. At that range, perhaps twenty yards, cannon fire could open a breach quickly, with the rubble falling into the ditch, and the attackers would bring up material as well to fill it in. Then an assault took place. If the neighboring bastion wTas still held by the defenders, it could provide a murderous fire into the assault troops as they worked their way across the rubble in the ditch and into the breach, where the defenders had also posted troops. Frequently the assault was driven back, perhaps again and again, and the besiegers had to resort to a passive siege of trying to starve out the defenders. The result was a decline in the speed of war, since sieges again took perhaps an entire campaigning season, and the quick sieges and battles that had marked the era of the French invasions of Italy became a thing of the past.

Palmanova and Philippeville. When an army arrived to besiege a well-fortified city, it rarely had the capacity to lay siege lines around the entire defensive perimeter. Instead the military engineers would decide which side would be most vulnerable or easiest to approach and concentrate the guns there. Once it was clear where the attackers were concentrating their efforts, the defenders moved guns and sup-plies to the bastions under attack from the others. It was often difficult, however, to move the gun carriages and their large horse teams through the narrow and crooked streets of a medieval town and up and down steep inclines. When governments decided to build entirely new fortified towns, as they did in several instances, the design of the new towns took into account that problem. Wide streets were laid out in a straight line that led to a central plaza, where the gun carriages could easily wait their turns to go to the bastions under attack. Inclines were made as gradual as possible. The Venetians, for example, built Palmanova northeast of their city in 1593 to protect it against Turkish attack from Hungary. They made it a real city in hope that the commerce attracted to it would help pay for the extensive and expensive fortifications that would be built. The enormous expense of the defensive works in the Italian-trace style forced the Venetians to reduce the number of bastions from twelve originally planned to nine. Cities such as Palmanova or Philippeville, which the Spanish began in 1555, reveal the precise geometry and symmetry of the Italian trace, especially when it was not complicated by the presence of existing defensive structures.

Other Improvements. By the late sixteenth century the offense again was catching up to the defense. In the 1540s German gunmakers working for Henry VIII discovered a better way to cast iron culverins. Iron being so much cheaper than bronze, the new method led to a dramatic increase in the number of quality guns that were capable of doing severe damage to fortifications. Steady improvements in siegecraft responding to the Italian trace reduced the initial advantage it had given to the defenders. Military architects responded by designing ways to push the besiegers’ heavy guns further from the fortifications by using outworks. The first outwork to appear was the ravelin. When attackers struck at a simple Italian trace, they usually concentrated their efforts on the curtain wall, not the bastions, which were far more solid and difficult to breach. The ravelin was a detached triangular bastion placed in front of the curtain wall. It appeared early in the development of the Italian trace but became a standard part of fortification only by 1580. Outworks developed further during the Dutch revolt against the Spanish (1566-1648). The primary theater of this long war was the border between the Netherlands and Belgium, a comparatively level but marshy region. It was difficult to dig deep ditches but it was easy to build extensive outworks in the area. Fortifications in this region began to use demilunes, which were smaller bastions placed in front of the main bastions, and horn and crown works that were built in front of the ravelins and demilunes. A fine example of the extended outworks that marked Dutch fortifications especially was Coevorden, designed by Maurice of Nassau, who was both an excellent military architect and a siegemaster. The inner line of defense consisted of the curtain wall and its bastions surrounded by a ditch 180 feet wide. Then came a line of ravelins and demilunes fronted by a ditch thirty feet wide. The outer line of defense was a set of horn and crown works with another ditch in front of them. Next came a glacis that extended for five hundred feet. When an enemy had finally worked his way up the glacis and dragged his heavy guns to the edge of the counterscarp of the ditch in front of the outer line, they were still more than eight hundred feet from the main defensive line, out of effective range for cannon of that era. Such a fortification required a great number of men to defend, but it also required that a besieging army have a far greater number. A siege also took a long time to win. It took the Spanish a constant siege of three years to take the Dutch-held port of Ostend on the coast of Belgium, partly because it was supplied by sea, which the Dutch controlled.

Significance. The Italian trace was a significant factor in the military successes of the European powers in the sixteenth century. It was particularly true of the Portuguese, who built and defended a vast trading empire with few men by using the new style fortification to defend their trading posts in the East, where they faced more powerful local forces than the Spanish did in the Americas. In India in 1571, a Portuguese fort with a garrison of about 1,100 men withstood a siege conducted by the local ruler with more than 100,000 men. Once the Spanish began to build similar forts to defend their ports in the Caribbean, the attacks by English pirates against port cities such as Havana were far less successful than they had been previously. On the other hand, the failure of the Ottoman Turks to keep up with European fortification design and siegecraft was a significant factor in the eventual decline of that empire’s power in the late sixteenth century.


Christopher Duffy, Siege Warfare: The Fortress in the Early Modern World 1494-1660 (London: Routledge & Kegan Paul, 1979).

Simon Pepper, Firearms and Fortifications: Military Architecture and Siege Warfare in Sixteenth-Century Siena (Chicago: University of Chicago Press, 1986).

Richard A. Preston, and others, Men in Arms: A History of Warfare and Its Interrelationships with Western Society (Fort Worth, Tex.: Holt, 1991).