The Diffused Fortress I

fig 4

In a postscript to his lesser-known work Histoire d’une Forteresse (1874), Eugène-Emmanuel Viollet-le-Duc (1814-1879) describes the diffusion of fortifications brought about by the introduction of ranged artillery. The morphology of fortified towns was redefined by the need to operate in increasingly large arenas of conflict.

Histoire d’une Forteresse recounts the history of a fictional town in a valley in eastern France. Positioned strategically, it undergoes a series of sieges and rebuildings illustrating the practice of fortification from Stone Age Gaulish tribes to the Napoleonic wars of the early nineteenth century. Some of the town’s seven sieges result in it’s capture, while in others the town is able to hold out against the invaders. Illustrated with the lucid metal engravings characteristic of Viollet-le-Duc’s books, the text describes military architecture as fundamentally a problem of tactically exploiting matter. Neither tactics nor constructed matter alone are able to keep the fortress secure or overrun it. Instead security consists in a strategic conjunction between matter (the physical matter of the fortress and the surrounding terrain) and military practices. The fortess is a practiced site.

The conclusion, which is the only part of the text we will be concerned with in this paper, purports to be extracted from the notes of one Captain Jean, dated 1871, a year in which French military confidence had been shaken by the Prussian seige of Paris. “Attack,” writes Jean,

“implies a shock or onset; defence is a resistence to this onset. Whether a piece of ordnance discharges a ball against a plate of iron, or a casing of masonry, or an earthwork; or an assaulting column climbs a breach, the problem is substantially the same; in either case we have to oppose to the impulsive force a resistance that will neutralise its effect.” (Viollet-le-Duc, 1876: 357)

Defensive strategy, tactics, and technology therefore, aim to oppose, resist, and neutralise the effect of shock, at whatever scale it comes. Jean’s brief essay uses a series of ten geometrical plan diagrams to describe a fundamental shift in the nature of fortification, brought about by the shock of artillery.

In the simplest case of attack, a series of blows are exchanged in one-to-one combat. Advantage can be taken by outnumbering the enemy, or alternatively, by surrounding oneself with an enclosure. Jean’s first drawing shows that a group protected in this fashion strictly limits the way attackers can engage them. No matter how the attackers configure themselves, or what numeric advantage they hold, they can only confront the defenders individually.

When ranged weapons, “projectile arms”, are taken into account, the attackers receive an advantage. They can set up a line of weapons which can converge their fire on a point on the circumference of the defenders. Because of the nature of a circular fortress, the defenders are extremely limited in their ability to converge fire on the attackers. They are constrainted to scatter their fire outwards, while the attackers can concentrate it inwards. To ameliorate this asymmetry, the fortress can be constructed with external appendages, salients, that allow the defenders to establish a broader base of fire, as well as offering better visibility of the foot of the main wall. In response, however, the attackers can reconfigure, beginning their assault by concentrating fire on the exposed, outflung salients. Defender and attacker compete for the ability to converge fire on the other while remaining protected. This is the central tension of fortification, which Jean writes,

“regulates and will always regulate attack and defense; distances alone modify its applications… The more eccentric the defence is, the more distant must be the attack, and the wider the perimeter it must occupy; but it should be observed, that the more widely the defence is extended, the more open its flanks are to attack.” (Viollet-le-Duc, 1876: 360).

The 17th century fortifications of Sebastien le Prestre de Vauban (1633-1707), based on the star-shaped forts of the Renaissance, work according to this principle [3]. By extending salients out from the main fort to form bastions, the defenders create a wider line of fire which can be concentrated at any point on a circumference around the fortress corresponding to the range of the artillery, while minimising the exposure of any individual salients to concentrated fire from attackers. In the fourth of his sequence of diagrams, the geometry of the star-shaped fort is seen to be partially material (in the form of the stone walls), and partially immaterial (in the form of sight-lines, firing ranges, and trajectories). These lines of force or agency form a coherent system with lines of solid material. The trajectories do not simply inform the walls, nor do the walls simply define the trajectories. These immaterial lines are as essentially architectural as rows of cut stone. Captain Jean notes that this worked very well for the ranges of the artillery in use in Vauban’s time, but as the range of artillery increases (he notes that late nineteenth-century artillery was effective at ranges of eight or nine kilometres), the balance swings again. In operations at these greater distances, the fortress is proportionally reduced to a point. The attacker, “taking advantage of an indefinite amount of space” is hidden and free to move, while the defender, “soon encumbered with débris of all kinds” is slowed and confused. Under these asymmetrical conditions of mobility, Jean says, the end result is never in doubt. The solution? “In proportion to the length of the trajectory, therefore, the defence must remove its defensive arrangement from the centre of the place.” (VIollet-le-Duc, 1876: 363).

In the sixth diagram in this section, Viollet-le-Duc re-plots Vauban’s geometry for these larger dimensions. Vauban’s star-forts presented fronts of about 360m, but the new situation called for fronts twelve or thirteen kilometres long. To achieve this, the salients are detached from the main fortress; instead of bastions, they are distributed as separate forts forming a defensive ring. They are positioned at the limit of their own firing range from the central fortress, so they themselves cannot be used to bombard it. The arrangement is then refined into a dual ring of forts, so that no one fort is exposed without other forts being able to provide supporting fire from the flanks. In the drawing, the solid material of the fortress appears as a small hatched mass embedded in an increasingly complex field of radii and circumferences demarcating the domains of projected firepower. Railways are necessary to connect the distant forts, and communication lines must be kept open. The territory surrounding the fortress proper becomes a zone of mobility.

In the eighth drawing, the solid material of the fortress walls is given no presence at all; the entire fortress has dissolved into arcs and radii, with certain key points marked algebraically. However, the fortress has not become obsolete, but ubiquitous. The ninth drawing reveals that each individual fort retains the articulation of salients and angles appropriate to smaller arms and closer quarters. The efficacy of the new system was no longer in the solidity of a fixed centrepoint but in the furnishing of a territory. In the tenth drawing, we are shown how this abstract geometry is deformed by the terrain itself. We see the fortress with its wall, but the arced lines of the wall no longer indicate the presence of solid material, as they did in the first drawings. The wall is the extent of the artillery’s range; it is a horizon of effect surrounding a zone of mobility rather than an enclosure.

[ This older post from David Gissen’s HTC Experiments makes an interesting connection between Viollet-le-Duc and conceptions of crowds in the nineteenth century. ]

[ Part II ]

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