The covert way, q, is a space of ground contained between the ditch and a parapet towards the country, x, called the glacis. The covered way affords the advantage of collecting troops to act against the enemy. It enables the besieged to move in safety along the whole of the counterscarp, and to pass to and from it. It furnishes a closer and more raking fire than that of the rampart. It protects by its elevation the revetments of the works, which the besiegers can only batter in breach, by establishing their batteries upon the crest of the glacis, where they are exposed to a plunging and close fire from the works.
In order to defend the covered way obstinately, and not be compelled to abandon its branches as soon as the enemy shall occupy its salient angels, the traverses t are placed at the distance of 30 yards from each other. Finally, to be able to bring together in safety numerous bodies capable of making sorties with effect, spacious places of arms, p, are made in the re-entering angles, and as the defence of covered ways consists, principally, in these re-entering places of arms, good reducts, r', are made within them with a ditch and revetments. These reducts, besides, cover the openings of the ditch, between the extremities of the tenail and the flanks of the bastions, and prevent the besiegers from establishing batteries upon the crest of the places of arms, that might annoy the flanks of the bastions, and a part of the curtain. A small flank is made to the reduct on the side next the half-moon, to cover part of the interior, as well as the stairway that leads to it from the ditch, and to take the besiegers in rear when they attack the breach of the half-moon. (Bousmard)
There are also salient places of arms, s, opposite the salient angles; they are not so large as, and are of much less advantage in the defence, than re-entering places of arms.
Ten yards in width is generally allowed to the covered way; if it were narrower, the movements of the troops that defend it would be impeded; if wider, it would admit the establishment of batteries in breach in it. This would be an advantage to the besiegers, who would be sheltered from the reverse and enfilading fires of the besieged, by means of the relief of the glacis and the traverses. Between the traverses and the glacis, a passage is left which is termed the Defile of the Traverse.
The glacis should be made sloping towards the country, so that every part of its slope may be seen by the soldiers placed upon the banquette of the body of the place; this would not be the case if it were too steep.
Sixty yards in breadth, and 1 of the rampart is usually 40 feet in width. A 24-pounder mounted upon a siege carriage, (which is the longest,) requires for its recoil about 24 feet, and the remaining space is necessary for the passage of ammunition wagons. The terreplein is the part of the rampart on which the besieged places himself and his artillery: the rampart is surmounted by another body of earth of less thickness, (20 feet at most,) called the parapet, which is intended to cover those upon its terreplein.
The height of the crest of the parapet of the body of the place, should be such, that a bullet fired from it may reach the third parallel, established by the besieger at the foot of the glacis, and may pass 4 feet above the crest of the covered way, so that the soldiers defending that work, may fire at the same time without being incommoded by the wind of the bullet.
“Of the various methods of facing the exterior side of the rampart, which is called the Scarp, that in masonry is to be preferred, particularly when it is at least 25 feet in height. At this height is requires ladders of such a weight, to scale it, as cannot easily be managed. An earthen scarp, with a wet ditch, has the fault of exposing the place to be taken by surprise in time of severe frosts. An earthen scarp, upon a dry ditch, is the worst of all, as it may at any time be scaled by cutting its palisades with a hatchet. In besieging it, it does not become necessary to establish batteries in breach, a passage of the ditch alone will be sufficient, as the natural slope of the earth of the rampart will permit it to be mounted.”
“An earthen counterscarp spares the besieger the long and dangerous labour of the descent of the ditch, and facilitates the capture of the whole covered way when once a part of it is taken”. (Bousmard)
Source: Lallemand, Henri Dominique: A Treatise on Artillery (New York 1820)