The trans-Rhenane frontier, which extends for over three hundred miles from Rheinbrohl on the Rhine to Eining on the Danube, was defended by the same methods, although in certain sections the forts were more widely separated and the garrison was proportionately weaker. There were also fewer troops within call immediately behind the frontier line. Here also, between the cohort forts, stone ‘Zwischenkastelle’ and ‘Wachttürme’ furnished additional safeguards.[338] This whole system of frontier defence has been much criticized, and the limitations and possibilities of these artificial barriers must be carefully determined.[339] To take the negative side first, they could not, of course, be defended against unexpected attack, like the walls of a town, unless the assailants were only a small raiding party numbering some twenty or thirty men. On the other hand, in spite of the parallel of the ‘Customs Hedge’ in India,[340] it seems unlikely that fiscal considerations played any large part in determining the government on their construction. They doubtless acted, when built, as a check on smuggling, but the expense of their maintenance would have been quite out of proportion to the value of the trade done with the German or British tribes. The first purpose which they served was to furnish a screen behind which patrols could march in comparative safety, both by day and night, and keep the whole line under constant surveillance. Thus the passing of a hostile force could be instantly reported by messenger or signal[341] to the nearest _castella_, whence detachments could at once start in pursuit. Secondly, whereas the defenders nearly always had a mounted force close at hand drawn either from the _alae_ or the numerous _cohortes equitatae_, the raiders would probably be unmounted, since their start would be lost if they delayed to fill up the ditch and make a gap in the barrier large enough for their horses.[342] This barrier, too, had to be crossed again in retreat, and presented a very serious obstacle to a force encumbered with booty. Indeed, the defenders might reserve the great part of their forces for this moment, as is recommended by Byzantine military writers describing similar conditions.[343]