The wall of a Roman station, between the gateways, was often flanked by a series of towers, each projecting from the wall in the form of a semicircle or rather more than a semicircle. This was the case in some of the large Gallo-Roman cities, like Autun.[14] While the rounded form of these towers made them difficult to undermine or batter down, their summits served as standing-ground for the large _ballistae_ or catapults, from which javelins or stones could be hurled upon the attacking force. Their projection at regular intervals made it possible for the defenders to command the whole line of wall between each pair of towers; so that the besiegers’ attack would necessarily be concentrated upon the towers themselves. At Pevensey (16), where the enclosure of the station is almost oval, and not, as usual, rectangular, in shape, there are remains of twelve solid round towers, including those which flank the south-western gateway. At Burgh Castle there are four towers in the east wall, two of which are angle-towers.[15] Owing to the scarcity of good building stone in the district, the walls at Burgh Castle are unfaced, and are built of flint with bonding courses of tiles; only the upper portions of the towers were bonded into the walls. A bed of concrete, with a platform of oak planks above, formed the foundation of the towers.[16] The angles of the wall of Roman York were strengthened by large polygonal towers. A large portion of one of these, a magnificent example of Roman masonry, remains; it formed the north-western angle of the city, and was hollow, with an internal as well as an external projection (17). No outward projections appear to occur upon the Roman wall or in the walls of its stations. The “mile-castles,” as already noted, are built against the inner side of the wall. At Cilurnum (13) and Borcovicus there are foundations of square towers against and inside the containing walls, while the angles of the stations are simply rounded off. The western part of the north wall of Borcovicus has also been doubled in thickness, apparently to give a safe foundation for a large catapult planted on the top of the wall. The thickening was accomplished, at a date later than the original building, by constructing an inner wall, and filling up the space between this and the outer rampart with clay. At Cilurnum, which appears to have been in existence before the great frontier wall was made, the original east and west gateways were left on the north side of the wall, which intersects the station. As they were thus insufficiently protected, they were filled up solid with masses of rubble, and were probably used as platforms for catapults,[17] smaller gateways being made on the south side of the wall. [Illustration: York; Multangular Tower] [Illustration: Borcovicus; Praetorium]