RETRENCHMENT, in Fortification, is a work constructed within another, in order to prolong the defence of the latter by impeding or preventing the formation of lodgments when the enemy has gained possession of it; or to afford protection to the defenders till they can retreat with safety or obtain a capitulation. In the latter respects the interior work is called by the French engineers a reduiL.
Every principal work in permanent fortification is provided with its retrenchment or reduit; and some of these. as the reduit of the ravelin, and of the re-entering places o: arms, are constructed at the same time as the work, itself. while others, as the retrenchments within a bastion, are generally executed but a short time before they are wanted.
In 1552, when Metz was besieged by Charles V, thv Duke of Guise, who commanded in the town, by constructing new ramparts within the old, as fast as the latter ware destroyed by the besiegers, succeeded at length in compelling the emperor to raise the siege; and at the siege of Car.dia (1666-1669), the Venetians raised a rampart from our curtain to the next in rear of the gorge of the bastion Si Andrea, so that, long after that bastion was breached and taken, the town continued to hold out. Such prolonged defences are now rare, and the governor of a fortress a considered as having fulfilled his duty if he do not surrender till a breach has been made in the rampart of th« enceinte; though if the bastion were retrenched, he tmgh: sustain an assault without any risk of being refused a cap. tulatioD, or of seeing the town given up to be plundered. Tn the event of the assailants gaining the top of thehrench, the defendants would be able lo retreat within the retrenchment, the flre from which might then be concentrated upon the enemy while confined within the comparatively narrow space between the faces of the bastion.
The kind of retrenchment proposed by Cormontaignc for the bastion of a fortress is a rampart or parapet extending across the interior of the work in a right line, or rather in the form of a tenaille. [X, Fortification, vol. x., p. 377.] Its extremities join the faces of the bastion at '20 or 30 yards in front of the shoulders, by which means the Hank is left finite free, so that all its artillery can be employed in the defence of the main ditch, and there is room between the retrenchment and the shoulder of the bastion for two guns, by which the interior of the ravelin and the ditch of iis reduit might be defended, if necessary, even after the enemy had made a lodgment in the bastion.
As the retrenchment in this situation is liable to be enfiladed by a battery of the be>iegers on the glacis before the collateral bastion, it is proposed that another should be formed in rear of the gorge of the bastion attacked; and as in this case there would be sufficient room, the retrenchment may be in the form of a front of fortification with a revetted scarp and counterscarp.
The ditch in front of a retrenchment, as at X [fortifiCation], is cut quite through the parapet of the bastion, in order to prevent the enemy, after the assault at the breach, from passing along the top of that parapet, and getting to the rear of the retrenchment. This opening of the parapet does not however go lower than the level of the cordon of the scarp revetment, in order to avoid diminishing the height of that scarp, and thus offering a facility to the enemy, should he attempt to escalade the work at that place. Coimonlaigne proposed to retrench small bastions by constructing within them cavaliers of the same form as the bastion itself, and having a command of 5 or 6 feet above it. The fire from this high parapet might give the work some advantages during the progress of the siege; but from its figure a cavalier appears to be less proper than a retrenchment in the form of a tenaille, for the defense of the lerreplein at the top of the breach.
It is evident that full bastions like A [fortification] must be more convenient for being retrenched than those which are of the kind called hollow, as B ; since less earth is wanted to raise the retrenchment to the required level, and the scarps are covered by the opposite side of the ditch from the view of the enemy at the top of the breach. The most simple manner of retrenching a hollow bastion would be that of retaining the rampart on the interior side, where it is usually lOor 12 feet high, by a wall, and cutting a deep ditch at the foot; then forming a traverse across the terreplein of the rampart on each face (at 20 or 30 yards from the salient angle of the bastion) with a ditch in front. The enemy, in gaining the top of the breach, would then find himself arrested by these obstacles, and exposed to the fire of the defenders, till ladders and the support of a large body of troops could be obtained. The bastions of Ciudad Rudrigo, Badajos, and St. Sebastian were retrenched in this manner when those fortresses were besieged by the British and their allies during the Peninsular war. Vauban, having observed that the ravelin was sometimes abandoned by the defenders previously to an expected assault, on account of the difficulty of retreating across the main ditch under a fire from batteries on the glacis before the bastions, constructed, in the interior of that work, another, which might afford the defenders protection till the coming night would permit them to retire in safety. This work was at first only a wall, pierced with loop-holes for musketry and covering the steps at the gorge; hut when Vauban increased the size of the ravelin, he made this reduit.or retrenchment, to consist of a rampart and parapet similar to those of the ravelin itself, as at Neuf Brisae; and Corraontaigne subsequently enlarged the work, so as to render it a second ravelin, as at Y [fortification]. This spacious retrenchment contributes much to the prolongation of tho defence of the ravelin, since it is capable of containing a large body of troops; and each of its Hanks can carry three guns, whose fire might be directed against the counter-battery at the salient of the bastion, or might serve for the defence of tho breach in the face of the latier, should the enemy attempt to make an assault before he has obtained possession of the reduit. But to take this last, it would be necessary to breach Us faces either by artillery or by mining; and the passage of its ditches would he difficult under a close fire from the ramparts near the shoulders of the neighbouring bastions. In order that the defenders might be able to retain possession of the extremities of the ravelin, near the main ditch, after the salient part may have been taken, retrenchments, or conpures, as they are called, similar to the traverses, /, /, &c., are recommended to be formed across the terreplein of the ravelin. Behind these some of the defenders may retire, and keep up a fire against the enemy's lodgment near Z.
When Vauban had enlarged the re-entering places of arms, L [bastion; Fortification], he retrenched the interior with stockades, which, by covering the steps leading from the ditches in their rear, protected the retreat of the defenders of the covered-way; and Coehorn appears, about the same time, to have constructed brick walls, provided with loop-holes, in the places of arms, at Bergen-op-Zoom, for the same purpose. But Cormontaigne, in older to render the defenre of the places of arms more obstinate, and to secure more effectually the retreat of the defenders of the covered-way, constructed reduils, as W [fortificaTion], with parapets of earth 12 or 14 feet thick; and he revetted the sides of their ditches, in order to diminish the risk of a surprise: he also gave them flanks, which he made perpendicular to the oovered-way, that a fire of light artillery or musketry might be directed from them against the enemy while attempting to crown the salients of the ravelin. These reduits moreover cover the shoulders of the bastions and the portions of the curtain which might be seen and breached by a fire directed between the flanks of the bastions and the tenailles; and their faces are directed so that they cannot be enfiladed. The crest of their parapet is about four feet higher than that of the glacis in their front.
The advanced works about a fortress are recommended to be retrenched with round-towers of masonry, provided with upper and lower tiers of loop-holes for musketry. These are called safety redoubts; and in Montalemberl's ' Fortification Perpendiculaire,' such towers are constructed, to carry artillery, within the rampart of the enceinte.
For retrenchments in field fortification, see Blockhouse.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)