Banquette, is an elevation, e f, behind the parapet, which runs along it, and which the infantry steps up on, to fire over the parapet; see genouillère.
Source: Rumpf, H. F.: Allgemeine Real-Encyclopädie der gesammten Kriegskunst (Berl. 1827)
Of the Banquette and Breast Height
The usual height of a parapet being 8 feet, and a soldier, of the middle size, firing with difficulty over a parapet of the height of 4 feet 6 inches, of the usual thickness and inclination to the horizon, it follows, that in order to fire he must be mounted upon a banquette of such an elevation, that the part m a (fig. 2) of the parapet, between its crest and the banquette a b, (which is called the breast height,) may be less than 4 feet 6 inches. It is usually made only 4 feet 3 inches. The height of the banquette of course varies with that of the parapet; but when they are meant to be permanent, 4 feet 6 inches is allowed, because the height will be diminished by the settling of the ground.
The greatest breadth of the top of the banquette is 3 feet, when there is to be no more than one rank of soldiers; and 4 feet to 4½ feet when there are to be two. This will give them room to manœuvre with ease. In those parts intended for cannon, the banquette must have from 12 to 21, or even 24 feet in thickness, to allow the recoil, and facilitate the management of the pieces. This will vary with the nature and length of the guns, as they may be either field pieces or heavy guns, with travelling or garrison carriages. There must be from 12 to 18 feet of epaulement to each piece. The breast height, for cannon, need not be higher than from 2 feet 6 inches, to 3 feet 10 inches; so that they may fire over it, it is then called genouillère, (fig. 4 m x, x u k). These are called Barbette Batteries. (See Construction of Batteries, vol. I.)
The slope of the banquette should be as great as possible; double its height at the very least, so that it may be easily mounted, and even descended by stepping backwards. It is sometimes cut into steps to diminish its breadth. (fig. 5 x)
Source: Lallemand, Henri Dominique: A Treatise on Artillery (New York 1820)