The Raetian frontier-fence was a mere barrier, produced by piling up quarry-stones; there were no fosses or watch-towers, and the forts, constructed behind the Limes without regular succession and at unequal intervals (none nearer than two and a half to three miles), stand in no immediate connection with the barrier-line. As to the order in time of the constructions there is no definite testimony; it is proved that the upper Germanic line of the Neckar was in existence under Pius,[85] that placed in front of it from Miltenberg to Lorch under Marcus.[86] The idea of a frontier-bar was common to the two structures, otherwise so different; the preference in the one case for the piling up of earth--whence the fosse for the most part resulted of itself--in the other case, for layers of stone, probably depended only on the diversity of the soil and of the materials for building. It was common to them, further, that neither the one nor the other was constructed for the defence, as a whole, of the frontier. Not merely was the hindrance, which the piling up of earth or stone presented to the assailant, slight in itself; but along the line we meet everywhere with commanding positions, morasses lying in the rear, a want of outlook towards the country in front, and similar clear indications of the fact, that in the tracing of it warlike purposes generally were not contemplated. The forts are of course arranged for defence, each by itself, but they are not connected by paved cross-roads; and so the individual garrison relied for support not on those of the neighbouring forts, but on the rear-base, to which the road led, whereby each was kept garrisoned. Moreover, these garrisons were not dovetailed into a military system of frontier defence; they were rather fortified positions for a case of need than strategically chosen for the occupation of the territory, as indeed the very extent of the line itself, compared with the number of troops at disposal, excludes the possibility of its defence as a whole.[87] [Sidenote: Object of these structures.]