By the end of the fourteenth century, it was apparent that gun-powder weapons were changing siege tactics and, consequently, fortification construction had to adapt to them. From shortly after their invention, guns began to be used in sieges. They were possibly used in 1338 at the siege of Cambrai, in 1340 at Tournai and Quesnoy, in 1342 at Rennes and Hennebout, and almost certainly at Calais in 1346–47. But on all of these occasions their use was rather limited, especially in comparison to other siege machines.
Gunpowder weapons continued to be used to attack fortifications throughout the fourteenth century, and by the end of the century they had begun to have some successes in breaching them. In 1374, the French used guns to bring down the town walls of Saint-Saveur-le-Vicomte, and in 1377, Philip the Bold, Duke of Burgundy, used them to penetrate the wooden walls of the fortress of Odruik. At the beginning of the fifteenth century, no castle or town wall was safe from bombardment, and castles, such as Berwick in 1405, and towns, such as Harfleur in 1415, seem to have been fairly easily taken by attackers who used gunpowder weapons. Up to this time, some sieges were brought to a conclusion more quickly by gunpowder bombardment. Where it once could have taken more than a year to capture a heavily fortified and well-stocked town or castle, it might now take less than one month to capture one. Perhaps the most famous defeat by bombardment in the early history of gunpowder weapons was the city of Constantinople, the great walls of which fell in 1453, some contemporary sources suggest, due to the continual and sustained attack.
Naturally, this threat of attack by gunpowder weapons influenced those responsible for the construction or maintenance of fortifications. Military leaders, as well as fortification architects and engineers, quickly realized that traditional medieval castles and town walls, with their tall, flat surfaces, made easy targets for guns, especially for the large-caliber bombards so frequently used in sieges of the period. The tall walls of medieval fortifications had not been built to sustain the continual barrage on a single area that the new gunpowder weapons could deliver, and in fact the relative thinness of the base of these walls, as well as the height of the walls, invariably made it easier to breach these fortifications.
It was both too expensive and too time-consuming to rebuild all fortifications to meet the attacks of gunpowder weapons. Therefore, the initial move was to outfit them with their own gunpowder artillery as a defense. Gunpowder weapons began to be delivered to castles and town walls by the middle of the fourteenth century, but guns mounted on the tops of castle and town walls made little sense as they could not effectively defend the wall below them, an area that was most likely to be attacked. This meant that the wall itself needed to be pierced with gunports so that defending fire could be more directly aimed at attacking artillery. This may have occurred as early as 1347, when the gate at the castle of Bioule was recorded as having been defended on its first floor by two men firing cannons, but even so, the idea did not become popular in England until at least 1365 and in France until after 1380. Germany, Italy, and Spain may not have constructed gunports in their fortifications until even later than this.
Once gunports had become popular in England and France, however, a large number of fortifications, both castles and town walls, received them. In England, gunports were added to Quarr Abbey on the Isle of Wight in 1365–66 (perhaps their earliest English use), to Queensborough Castle in 1373, to Assheton’s Tower at Porchester in 1379, to Carisbrooke Castle in 1379–80, to the Canterbury town wall in 1380, to Cooling Castle in 1381, to Southampton Castle in 1382–86, to Saltwood Castle in 1383, to the Norwich town wall in 1385, to Bodiam Castle in 1386, and to the Winchester town wall in 1390. In France, gunports were added more slowly and in smaller numbers. Besides those that may have existed at Bioule Castle, it is only the gunports in the town wall of Mont-Saint-Michel and at the castles of Blanquefort and Saint-Malo that can be confirmed to have been built before 1400. Others were not built until after 1412, with the towns of Paris and Rennes receiving their gunports in 1415 and 1418, respectively. However, gunport construction in France increased through the end of the Middle Ages, while in England it declined drastically after 1420.
In the late fourteenth century most gunports were shaped like inverted keyholes with circular openings for guns below vertical slits. This may indicate that they were initially nothing more than arrow slits adapted for gunpowder weapons by adding a circular opening to the bottom, as fifteenth century gunports were usually built without the vertical slit; however, some architectural historians contend that the slits were there merely to facilitate the sighting of the weapon. Gunports were also quite small: the slits ranged in length between 381 and 813 millimeters and in width between 65 and 152 millimeters, while the circular openings ranged between 127 and 305 millimeters in diameter. Although some gunports were built higher in towers, especially after 1400, most were built near to the ground, generally no higher than 1 meter from the inside floor.
The number and distribution of gunports around the fortifications varied greatly. Sometimes there were very few gunports in a fortress, perhaps only one or two, while other fortifications, especially those built in the fifteenth century, contained a much larger number. For example, Raglan Castle, built c.1450, contained no fewer than 32 gunports. Most also were located in the fortification’s gates and towers. This allowed them to provide some flanking fire along the wall and causeways, although this may have been of limited effect, as most gunports allowed less than a 45-degree angle of fire.
The small size of these gunports meant that the large guns could not be used for defense of the fortification. This can also be seen in the small size of the gunport embrasures on which these guns were mounted, most of which were less than 70 centimeters long. This allowed only the smallest of mounted gunpowder weapons or handheld guns to be fired from these gunports. Still, gunports alone could not supply sufficient defense against most gunpowder artillery bombardments, especially as those guns had become much more powerful and accurate by the beginning of the fifteenth century and even more by the beginning of the sixteenth century. Other methods of improving defense were needed.
Some towns and castles tried to meet this threat by thickening their walls with piles of earth behind them. While this did add protection to the walls so that they could not be easily breached in some instances, it was not always successful. The earthen rampart exerted a heavy pressure on the wall and weakened the masonry instead of strengthening it. As Philip, Duke of Cleves, noted at the end of the fifteenth century, “whenever the guns batter the wall, the earth tumbles down with the masonry, which makes it all the easier for the enemy to climb into the breach.” Other walls were thickened with more masonry instead of earth. Or a separate, shorter wall, known as a faussebray, was added slightly in front of the main walls, usually no more than two to three meters, to take the impact of gunshots that might hit the more vulnerable base of the walls. Finally, some walls had a sloping glacis of masonry added to their front to deflect gunshots, avoiding direct impact on the flat wall. However, these additions were often very expensive and therefore not generally obtainable for many towns or castle owners whose resources were often limited. Still others tried to increase the size of the ditches surrounding their fortifications, noting the relative security against bombardment of large moated fortresses, such as Bodiam, Kenilworth, or Caerphilly castles. The most impressive increase of this sort was at Rhodes where, between the Ottoman Turkish sieges of 1480 and 1522, the moat was doubled in size, from roughly 23 meters to more than 50 meters. In fact, in three sizeable places along the moat at Rhodes the size was increased so much that islands of earth made from, on average, 10 meters of the former moat edges were left as additional fortifications, tenailles, between the walls and the new moat edges. But these were even more expensive than most townspeople or castle owners could afford.
Some builders tried to increase the defense of castles and town walls by adding new fortifications to the existing defensive structures and then filling them with defensive gunpowder artillery and missile troops. In essence the theory behind these was the same as that of gunports, facing guns with guns, except that they were entirely separate from the castles and town walls themselves, and thus their defeat did not necessarily mean the collapse of a fortification. Generally, these were built in two styles. The first style was the low earthwork defense, known as a boulevard, which was typically placed before a vulnerable gate or wall. Its defense derived from its large number of guns (which increased the amount of defensive firepower), its low height (which made it easier to fire), and its earthen walls (which more readily absorbed the impact of stone and metal cannonballs). It seems to have been particularly popular in mid-fifteenth-century France among fortresses that were more open to gunpowder artillery bombardment. Perhaps the most famous medieval boulevards were those that stood outside the fortified bridgehead called the Tourelles, at Orléans, which was attacked first by the English, led by Thomas Montagu, earl of Salisbury, in 1428, and then by the French, led by (among others) Joan of Arc, in 1429. Built in 1426, these boulevards were filled with gunpowder weapons and missile troops. Both fell, but only after numerous assaults. According to the contemporary Journal du siège de Orléans, in order for Joan of Arc’s soldiers to capture the boulevard, they made “many marvelous assaults, during which many marvelous feats of arms were performed … [and] many Frenchmen were killed or wounded.”
The second style was the artillery tower. This was usually newly constructed tower, again filled with a large number of gunpowder weapons, which was added to the most exposed part of a fortress. Its purpose was nearly the same as the boulevard, both to increase the amount of defensive firepower and to add flanking fire to a vulnerable wall or gate, but it was generally much taller and constructed of stone. The artillery tower was also usually round in shape to provide no flat surfaces to enemy gunfire. It soon became the preferred artillery fortification added to castles and town walls, and was in fact built well into the early modern period. (Boulevard construction had ceased for the most part by the end of the fifteenth century).
Certainly the most varied, if not also the most numerous, artillery towers can be found at Rhodes in the eastern Mediterranean. Following their successful defense of the city in 1480, the Knights Hospitaller, who controlled the city, were determined to add artillery towers at every angle and vulnerable spot along the landward sides of the walls. (The seaward side was guarded by three fortresses at the end of moles that jutted into the harbor, these having received their own updating and rebuilding, and by heavy chains running between them.) The result over the next 40 years was the construction of more than 10 new towers and one new gate, the reconstruction of 3 older gates, and the refitting of all the other towers to withstand the gunpowder artillery that the Hospitallers were certain would be the result of a second Ottoman siege. These included the massive St. George bastion, built originally in 1481 under the direction of Grand Master Pierre d’Aubusson, and rebuilt in 1496 by that same Grand Master, who had recently been appointed cardinal of Asia. This huge five-sided structure was topped by a large number of artillery platforms facing in all directions, from which the largest of gunpowder weapons could fire at opponents on the moat opposite them. Inside the bastion were two stories of chambers filled with gunports. From these, medium-sized and small guns could be fired down the moat. Perhaps because of its size and the number of guns that the St. George bastion contained, it was not attacked during the 1522 siege, when the Ottoman Turks finally did return. This time they captured Rhodes; large artillery towers and numerous guns simply could not compensate for the seemingly endless supply of Turkish troops and artillery or the determination of their Sultan, Suleyman I the Magnificent.
Yet none of these additions to existing fortifications provided complete security against gunpowder artillery attack, as was made clear at Rhodes in 1522, and by the end of the fifteenth century it was recognized that traditional medieval fortifications, even with a number of additions, could not provide adequate defense against a gunpowder weapon attack on their inhabitants. A more elaborate system of fortifications was needed, a system where walls could withstand the constant impact of stone or metal cannonballs while at the same time offering its own gunpowder artillery bombardment against those besiegers.
Known as the trace italienne, because of its appearance in numerous early-sixteenth-century Italian city-states, this system of artillery fortifications may have originated in Leon Battista Alberti’s De re aedificatoria (On Construction), written in the 1440s (although it was not popular until after its 1485 printing). Alberti contends:
1) that fortification walls facing gunpowder weapons should be both short enough to easily see the ground below them and wide enough to withstand the impact of cannonballs;
2) that artillery towers projecting at an angle beyond the walls should be added to the fortification—this would not only protect the fortification itself but also keep offensive guns at bay and cover blind spots along the fortress walls;
3) that angled bastions projecting out at regular intervals from the fortress walls be built giving increased flanking cross-fire along the surface of those walls;
4) that as time passed further refinements should be added to the fortification: wide and deep ditches along the walls to keep enemy artillery at a distance and to cut down on mining with detached casements or bastions (called ravelins) built beyond those ditches to further impede enemy artillery attacks; and
5) that extensions should be built to these fortifications, complete with crownworks or hornworks, to protect outside strategic areas.
The trace italienne completely changed the future of fortification construction. Few tall castles and town walls would be newly built as defenses against foreign invasion or domestic rebellion. Already by the end of the fifteenth century, the Italian fortifications of Sarazana, Avezzano, and Ostia Antica had begun to adapt Alberti’s ideas in their construction. However, it was not until after 1494, when Charles VIII’s invasion of Italy would show the vulnerability of medieval fortifications to artillery bombardment, that most trace italienne artillery fortifications began to appear, and they would continue to be built into the nineteenth century.