Until the invention of gunpowder, or rather till that of artillery (Fig. 57) the whole art of fortification, says the learned Prosper Mérimée, consisted in following more or less exactly the traditions handed down by the Romans. The stronghold of the Middle Ages had precisely the same characteristic as the ancient _castellum_. The methods of attack against which the engineers had to guard were the assault by escalade, either by surprise or by force of numbers, and the breach, caused either by sapping, mining, or by the battering-rams of the besiegers. The employment of machines or _engines_ of this description was much less frequent after than before the fall of the Roman empire, when the art of war knew no higher flight than to lay siege to a place or sustain a siege. [Illustration: Fig. 57.--Mortars on Movable Carriages.--From an Engraving in the “Kriegsbuch” of Fronsperger: in folio, Frankfort, 1575.] The first operation of the besiegers was to destroy the outworks of the besieged place, such as the posterns, the barbicans, the barriers, &c. As most of these outworks were built of wood, attempts were generally made to cut them to pieces with hatchets, or to set them on fire with arrows to which were fastened pieces of burning tow steeped in sulphur, or some other incendiary composition. [Illustration: Fig. 58.--Caltrop, or Crow’s-foot (Fourteenth Century).] If the main body of the place were not so strongly fortified as to render a successful assault by force impossible, it was usual to attempt an escalade. With this end in view, the moat, which was generally literally strewn with caltrops (Fig. 58), was filled up with fascines, on which ladders were reared against the ramparts, while archers on the brink of the ditch, protected by mantlets stuck in the ground, drove away with their arrows any of the defenders who attempted to show themselves above the parapets or at the loopholes. [Illustration: Fig. 59.--Rolling Tower for scaling the Walls of Towns.--Miniature from the “Histoire du Monde,” Manuscript of the Fourteenth Century (Library of M. Ambroise Firmin-Didot).]