Bastion

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In ancient fortification, the walls of towns were flanked by round or square towers, from which archers and war machines could direct their projectiles on the storming enemy while he was held in check by the ditch. On the introduction of artillery into Europe, these towers were made considerably larger, and ultimately, in the beginning of the 16th century, the Italian engineers made them polygonal instead of round or square, thus forming a bastion. This is an irregular pentagon, one side of which is turned inward toward the tower, so that the opposite salient angle faces the open field. The 2 longer sides, enclosing the salient angle, are called the faces; the 2 shorter ones, connecting them with the town wall or rampart, are called the flanks. The faces are destined to reply to the distant fire of the enemy, the flanks to protect the ditch by their fire. The first Italian bastions still showed their descent from the ancient towers. They kept close to the main walls; the salient angle was very obtuse, the faces short, arid the parapet revetted with masonry to the very top. With such ‘small bastions, the main office of the flank was the defence of the ditch in front of the curtain connecting 2 bastions; consequently, the flanks were placed perpendicular to the curtain. These bastions were distributed either on the angles of the polygon forming the whole enceinte of the fortress, or where one side of the polygon was so long that a part was not within effective musket range of the 2 projecting flanks, an intermediate bastion, called piatta forma, was erected on its middle.

With the improving siege artillery of the 17th century, larger bastions became necessary, and very soon the curtain lost its importance, the bastions being now the principal points to be attacked. The office of the flanks was also changed: they now had to enfilade, chiefly, the ditch in front of the face of the opposite bastion, and instead of being erected perpendicular to the curtain, they were made perpendicular to the prolongation of that face, called the line of defence. The height of the masonry revêtement was reduced so as to be covered from direct fire by the glacis or the parapet of the lower outworks. Thus bastions, in the hands of the old French and German school, and subsequently in those of Vauban and Coehorn, underwent many changes of form and size, until about 1740, Cormontaigne published a system of bastionary fortification which is generally considered as the most perfect of its kind. His bastions are as large as they can well be made; his flanks are nearly, but not quite, perpendicular to the lines of defence, and great improvements are made in the outworks.

Bastions are either full or empty. In the first case, the whole of the interior is raised to the height of the rampart; in the latter, the rampart goes round the interior side of the bastion with a sufficient breadth for serving the guns, and leaves a hollow in the middle of the work. In full bastions, cavaliers are sometimes erected: works, the sides of which run parallel with those of the bastion, and are elevated high enough to allow of the guns being fired over its parapet. From the commanding height of such cavaliers, guns of the greatest range are generally placed in them in order to annoy the enemy at a distance.

The system of fortification based upon bastions was the only one known from the 16th to the end of the 18th century, when Montalembert put forward several new methods without bastions, among which the polygonal or caponniere system for inland fortresses, and the system of casemated forts with several tiers of guns, have found most favor.