OISEAU, in the fifteenth and sixteenth centuries, a small masonry work which, disposed at the salient angles in the ditch, swept the latter and was intended to bar the passage. The _oiseaux_ were masked by the counterscarp. In modern polygonal fortifications this plan has been re-adopted (see p. 373). OPPIDUM (Latin), a citadel or fortified strategical position, among the Gallic populations. Many Roman camps were formed at the epoch of the conquest of Gaul, on the Gallic _oppida_, which were only a kind of intrenched camps formed upon elevated plateaux. Several of our French towns occupy the sites of ancient _oppida_--Langres, Laon, Béziers, Carcassone, Uzerche, Sainte-Reine (Alesia), Le Puy-en-Velay, Semur-en-Auxois, Avalon, Puy d'Issolu, &c. ORILLON, projecting part of faces of bastions, intended to mask the flanks and to shield the guns which arm these flanks (see p. 280). PARADOS, mound of earth disposed behind the guns in battery, to shelter them as well as the servers from reverse firing. PARALLEL, trench parallel to the sides of the polygon of a fortress, supplying a covered communication for planting and serving the siege batteries. Formerly these parallels were required for approaching and planting the breach batteries. These parallels communicated with each other by trench boyaux traced in zigzags, so as not to be raked by the fires of the place (see p. 330). PARÉCLAT, epaulment of earth, or formed with gabions raised on the ramparts, or in the middle of bastions, to shelter the defenders from the splinters of bombs and shells. PLACE D'ARMES, space defended by an epaulment, intended to enclose a body of men and to protect it from projectiles (see pp. 288, 307). POSTERN, secondary gate, small gate, generally masked (see p. 170). RAMPART, epaulment raised with the earth taken from the ditch sunk on the outer side; also a wall crowned with a parapet and rampart walk. It signifies a permanent defence. RAVELIN, name originally given to demi-lunes. A work consisting of two faces, open at the gorge, low, and intended to sweep the exterior between two bastions (see p. 279). REDAN, work presenting a salient angle and a re-entering angle (see p. 305). RETRENCHMENT, a work made to augment the defensive strength of a place within the permanent fortifications, so as to present a fresh obstacle should the latter fall into the power of the enemy; the retrenchment consists of an epaulment of earth raised with the material from the ditch sunk on the outside. STIMULUS (Latin), barbed iron crook, which, fixed on a short stake stuck in the ground or in the bottom of conical holes, protected the approaches of a defence (see p. 80). TENAILLE, work consisting of a curtain having at each extremity two demi-bastions (see pp. 278, 306). TRAVERSE, a mound of earth disposed across covered ways, _terre pleins_, bastions, and curtains, to shield the guns and the defenders against enfilade, oblique, or ricochet fire (see p. 335).