Fortification plan of Coevorden
, laid out in a radial pattern within polygonal fortifications and extensive outer earthworks as reconstructed in the early seventeenth century by Maurice, Prince of Orange
Their predecessors, medieval fortresses, were usually placed on high hills. From there, arrows were shot at the enemies, and the higher the fortress was, the further the arrows flew. The enemies' hope was to either ram the gate or climb over the wall with ladders and overcome the defenders. For the invading force, these fortifications proved quite difficult to overcome, and accordingly, fortresses occupied a key position in warfare.
Passive ring-shaped (enceinte) fortifications of the Medieval era proved vulnerable to damage or destruction by cannon fire, when it could be directed from outside against a perpendicular masonry wall. In addition, an attacking force that could get close to the wall was able to conduct undermining operations in relative safety, as the defenders could not shoot at them from nearby walls. In contrast, the bastion fortress was a very flat structure composed of many triangular bastions, specifically designed to cover each other, and a ditch. In order to counteract the cannonballs, defensive walls were made lower and thicker. To counteract the fact that lower walls were easier to climb, the ditch was widened so that attacking infantry were still exposed to fire from a higher elevation, including enfilading fire from the bastions. The outer side of the ditch was usually provided with a glacis to deflect cannonballs aimed at the lower part of the main wall. Further structures, such as ravelins, tenailles, hornworks or crownworks, and even detached forts could be added to create complex outer works to further protect the main wall from artillery, and sometimes provide additional defensive positions. They were built of many materials, usually earth and brick, as brick does not shatter on impact from a cannonball as stone does.
Bastion fortifications were further developed in the late fifteenth and early sixteenth centuries primarily in response to the French invasion of the Italian peninsula. The French army was equipped with new cannon and bombards that were easily able to destroy traditional fortifications built in the Middle Ages. Star forts were employed by Michelangelo in the defensive earthworks of Florence, and refined in the sixteenth century by Baldassare Peruzzi and Vincenzo Scamozzi. The design spread out of Italy in the 1530s and 1540s.
It was employed heavily throughout Europe for the following three centuries. Italian engineers were heavily in demand throughout Europe to help build the new fortifications. The late-seventeenth-century architects Menno van Coehoorn and especially Vauban, Louis XIV's military engineer, are considered to have taken the form to its logical extreme. "Fortresses... acquired ravelins and redoubts, bonnettes and lunettes, tenailles and tenaillons, counterguards and crownworks and hornworks and curvettes and fausse brayes and scarps and cordons and banquettes and counterscarps..."
The star-shaped fortification had a formative influence on the patterning of the Renaissance ideal city: "The Renaissance was hypnotized by one city type which for a century and a half—from Filarete to Scamozzi—was impressed upon all utopian schemes: this is the star-shaped city." In the 19th century, the development of the explosive shell changed the nature of defensive fortifications. Elvas, in Portugal is considered by some to be the best surviving example of the Dutch school of fortifications.
When the newly-effective maneuverable siege cannon came into military strategy in the fifteenth century, the response from military engineers was to arrange for the walls to be embedded into ditches fronted by earthen slopes (glacis) so that they could not be attacked by destructive direct fire and to have the walls topped by earthen banks that absorbed and largely dissipated the energy of plunging fire. Where conditions allowed, as in Fort Manoel in Malta, the ditches were cut into the native rock, and the wall at the inside of the ditch was simply unquarried native rock. As the walls became lower, they also became more vulnerable to assault.
The rounded shape that had previously been dominant for the design of turrets created "dead space", or "dead" zones (see figure), which were relatively sheltered from defending fire, because direct fire from other parts of the walls could not be directed around the curved wall. To prevent this, what had previously been round or square turrets were extended into diamond-shaped points to give storming infantry no shelter. The ditches and walls channeled attacking troops into carefully constructed killing grounds where defensive cannon could wreak havoc on troops attempting to storm the walls, with emplacements set so that the attacking troops had no place to shelter from the defensive fire.
A further and more subtle change was to move from a passive model of defence to an active one. The lower walls were more vulnerable to being stormed, and the protection that the earthen banking provided against direct fire failed if the attackers could occupy the slope on the outside of the ditch and mount an attacking cannon there. Therefore, the shape was designed to make maximum use of enfilade (or flanking) fire against any attackers who should reach the base of any of the walls. The indentations in the base of each point on the star sheltered cannons. Those cannons would have a clear line of fire directly down the edge of the neighboring points, while their point of the star was protected by fire from the base of those points. The evolution of these ideas can be seen in transitional fortifications such as Sarzana in northwest Italy.
Ideal fortified city: a plan of Nové Zámky
(Neuhäusel) in Slovakia
, built in 1663, drawn c. 1680
Thus forts evolved complex shapes that allowed defensive batteries of cannon to command interlocking fields of fire. Forward batteries commanded the slopes which defended walls deeper in the complex from direct fire. The defending cannon were not simply intended to deal with attempts to storm the walls, but to actively challenge attacking cannon and deny them approach close enough to the fort to engage in direct fire against the vulnerable walls.
The key to the fort's defence moved to the outer edge of the ditch surrounding the fort, known as the covered way, or covert way. Defenders could move relatively safely in the cover of the ditch and could engage in active countermeasures to keep control of the glacis, the open slope that lay outside the ditch, by creating defensive earthworks to deny the enemy access to the glacis and thus to firing points that could bear directly onto the walls and by digging counter mines to intercept and disrupt attempts to mine the fort walls.
Compared to medieval fortifications, forts became both lower and larger in area, providing defence in depth, with tiers of defences that an attacker needed to overcome in order to bring cannon to bear on the inner layers of defences.
Firing emplacements for defending cannon were heavily defended from bombardment by external fire, but open towards the inside of the fort, not only to diminish their usefulness to the attacker should they be overcome, but also to allow the large volumes of smoke that the defending cannon would generate to dissipate.
Fortifications of this type continued to be effective while the attackers were armed only with cannon, where the majority of the damage inflicted was caused by momentum from the impact of solid shot. Because only low explosives such as black powder were available, explosive shells were largely ineffective against such fortifications. The development of mortars, high explosives, and the consequent large increase in the destructive power of explosive shells and thus plunging fire rendered the intricate geometry of such fortifications irrelevant. Warfare was to become more mobile. It took, however, many years to abandon the old fortress thinking.