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 lodgment/ 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 instances the nterior work la called by the French engineers a red mit.
Every principal work in permanent fortification is provided with its retrenchment or redout ; and some of these, as the redout of the ravelin, and of the re-entering ',Laces; of 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 15:12, when Metz was besieged by Charles V., the Duke of fiulae, mho commanded in the town, by constructing new ramparts within the oh!. as fast, as the latter were destroyed by the besiegers, succeeded at length in compelling the emperor to raise the siege ; and at the siege of Candia (166G-1609), the Venetian* raised a ram part from one curtain to the next in rear of the gorge of the bastion St. Andrea, so that, long after that bastion was breached and taken, the town continued to hold out. Such prolonged defences are now rare, and Ow governor of a fortress is considered as having fulfilled his duty if he do not surrender till a breach has been made in the rampart of the enceinte; though if the bastion were retrenched, he might sustain an assault without any risk of being refused a capitula tion. or of seeing the town given up to be plundered. In the event of the assailants gaining the top of the breach, the defendants would be able to retreat within the entrenchment, the fire from which might then be concentrated upon the enemy while confined within the com paratively narrow apace between the faces of the bastion.
The kind of retrenchment proposed by Cormontaingne 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.] Its extremities join the faces of the bastion at 20 or 30 yards in front of the shoulders, by which means the flank is left quite free, so that all its artillery can be employed in 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 its 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 besiegers on the gleeis 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 in 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.
Cormontaingne 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 eiege ; but from its figure a cavalier appears to be less proper than a retrenchment in the form of a tenaille, for the defence of the terre plein 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 a; since less earth is wanted to raise the retrench ment to the required level, and the scarps aro 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 10 or 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 Rodrigo, Badajos, and St. Sebastian were retrenched in this manner when those fortresses were besieged by the British and their allies during the Peninsular war.