Final Bastion
A fortification style developed in the early modern period to counter gunpowder weapons.
A bastion fort or trace italienne (a phrase derived from non-standard French, meaning 'Italian outline') is a fortification in a style developed during the early modern period in response to the ascendancy of gunpowder weapons such as cannon, which rendered earlier medieval approaches to fortification obsolete. It appeared in the mid-fifteenth century in Italy. Some types, especially when combined with ravelins and other outworks, resembled the related star fort of the same era.
Quick Facts
- Type
- Bastion fort; trace italienne
- Primary inhabitants
- Military garrisons and defenders
- Status
- Historical fortification style, rendered obsolete by explosive shells in the nin
Lore & Background
The design of the fort is normally a polygon with bastions at the corners of the walls. These outcroppings eliminated protected blind spots, called 'dead zones', and allowed fire along the curtain wall from positions protected from direct fire. Many bastion forts also feature cavaliers, which are raised secondary structures based entirely inside the primary structure. Their predecessors, medieval fortresses, were usually placed on high hills. From there, arrows were shot at the enemies. The enemies' hope was to either ram the gate or climb over the wall with ladders and overcome the defenders. Passive ring-shaped (Enceinte) fortifications of the Medieval era proved vulnerable to damage or destruction when attackers directed cannon fire onto perpendicular masonry 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. 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.
Reader's Guide
When the newly-effective manoeuvrable 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', which were relatively sheltered from defending fire, because direct fire from other parts of the defences could not be directed around curved walls. To prevent this, what had previously been round or square turrets were extended into diamond-shaped points to eliminate potential cover for attacking troops. The ditches and walls channelled the attackers into carefully constructed zwinger, bailey, or similar 'kill zone' areas where the attackers had no place to shelter from the fire of the defenders. 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 on the outer edge of the ditch and against any 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 neighbouring 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. 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. Bastion forts were very expensive. Amsterdam's 22 bastions cost 11 million florins, and Siena in 1544 bankrupted itself to pay for its defences. For this reason, bastion forts were often improvised from earlier defences. Medieval curtain walls were torn down, and a ditch was dug in front of them. The earth used from the excavation was piled behind the walls to create a solid structure. While purpose-built fortifications would often have a brick fascia because of the material's ability to absorb the shock of artillery fire, many improvised defences cut costs by leaving this stage out and instead opting for more earth. Improvisation could also consist of lowering medieval round towers and infilling them with earth to strengthen the structures. It was also often necessary to widen and deepen the ditch outside the walls to create a more effective barrier to frontal assault and mining. Engineers from the 1520s were also building massive, gently sloping banks of earth called glacis in front of ditches so that the walls were almost totally hidden from horizontal artillery fire. The main benefit of the glaces was to deny enemy artillery the ability to fire point-blank. The lower the angle of elevation, the higher the stopping power.
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