Thus these extensive military structures had not, like the Britannic wall, the object of checking the invasion of the enemy. The intention rather was, that, like the bridges over the river-frontier, so the roads on the land-frontier should be commanded by the forts, but in other respects, like the river as the water-boundary, so the wall on the landward should hinder the uncontrolled crossing of the frontier. Other uses might be combined with this; the preference, often apparent, for the rectilineal direction points to its application for signals, and occasionally the structure may have been used directly for purposes of war. But the proper and immediate object of the structure was to prevent the crossing of the frontier. The fact, withal, that watch-posts and forts were erected, not on the Raetian but on the upper Germanic frontier, is explained by their different relations to the neighbours, in the former case to the Hermunduri, in the latter to the Chatti. The Romans in upper Germany did not confront their neighbours as they confronted the Highlanders of Britain, in whose presence the province was always in a state of siege; but the repulse of predatory invaders as well as the levying of the frontier-dues demanded at any rate ready and near military help. The upper German army, and in keeping with it the garrisons on the Limes, might be gradually reduced, but the Roman _pilum_ could never be dispensed with in the land of the Neckar. It might, however, be dispensed with in presence of the Hermunduri, who, in Trajan's time, alone of all the Germans, were at liberty to cross the frontier of the empire without special control and to trade freely in the Roman territory, especially in Augsburg, and with whom, so far as we know, border-collisions never took place. There was thus at this period no occasion for a similar structure on the Raetian frontier; the forts north of the Danube, which can be shown to have subsisted already in Trajan's time,[88] sufficed here for the protection of the frontier and the control of frontier-intercourse. This accords with the observation that the Raetian Limes, as it stands before our eyes, corresponds only with the more recent upper Germanic barrier-line perhaps laid out for the first time under Marcus. Then occasion for it was not wanting. The wars of the Chatti, as we shall see (p. 161), seized at this time also on Raetia; the strengthening too of the garrison of the province might reasonably stand in connection with the erection of this Limes, which, however little it was arranged for military ends, was at any rate doubtless constructed with a view to its being a frontier-bar, though of less strong character.[89] [Sidenote: Their effect.]