Parallel with the interior town wall there ran an exterior lower one, which, together with the former, enclosed a space, to which we have already referred, varying from fifty to twenty feet in breadth. We know very little about the _original_ height and form of this exterior wall. It suffered many changes and can no longer be traced in its original shape. Experts hold diametrically opposite views both as to the use and the height of it. But that is the way of experts. We shall probably not be far wrong in concluding that this wall was originally a mere crenelated crowning[31] of the escarp of the ditch; that catapults were worked from the space enclosed by the two walls; and that the chief object of the outer wall and the enclosure was to prevent the enemy from working at the main, or inner, wall and towers with his rams and moveable turrets. Later, when the use and effectiveness of artillery developed and guns supplanted catapults in vigour as well as in fact, some time at the end of the fifteenth or the beginning of the sixteenth century, we may suppose that this old crenelated wall was removed, and the escarp wall of the ditch was raised and strengthened and provided with embrasures for large cannon, and rounded off on the outside so as to neutralise the effect of shot striking the face of the walls. In this form the exterior wall is well preserved, and can be seen at many places in the course of a walk round the outside of the town. At many points in the circumference, but chiefly where the fortifications are accessible (_e.g._ near the Frauen Thor) the parapets of this curtain-wall present a somewhat remarkable arrangement. The parapets, pierced with embrasures for cannon, are surmounted by timber hoards or filled in with brick and mortar, like the old English half-timbered houses. In these hoards (wooden galleries roofed in with tiles) arquebusiers and even archers, who were still employed at that period, might be placed. Pieces in battery were covered by these hoards just in the same way as pieces in the "'tween decks" of a man-of-war. The crenelles of the hoards were closed by shutters opening on the inside, in such a way as to present an obstacle to the balls or arrows fired by the assailants placed on the top of the glacis. The outer, like the inner wall was provided with towers. These were thicker in construction but lower and less numerous than the interior ones. They were placed at intervals of 200 to 250 feet and amounted in all to forty or thereabouts. The chief purpose of them was to flank and command the ditch and thus to prevent the enemy from building a dam across it. With this object they projected some distance into the ditch.