In provinces whence archaeological evidence is not forthcoming, inscriptions indicate the same system. From Spain, for instance, we have an early inscription referring to an officer who held command over four cohorts,[328] and a similar brigade of three cohorts appears at Syene in Egypt in the reign of Trajan.[329] On the Danube frontier von Domaszewski has concluded from the epigraphical evidence that Aquincum and Arrabona each held two alae in the first century.[330]
This period of expansion may be considered to end with Trajan’s annexation of Dacia and his failure a few years later to execute a similar forward move on the Eastern frontier. With the accession of Hadrian a new policy begins, which advertised by the elaborate character of its defensive measures that the imperial government was firmly determined to renounce all further schemes of aggression, a determination which was adhered to until the power of decision lay no longer in Roman hands. The outward signs of this new spirit were the abandonment of the old _hiberna_, and the removal of their garrisons to stone forts of a new type, each arranged to hold no more than a single unit, which were placed at more or less regular intervals along the frontier instead of behind it.[331] The auxilia, that is, were transformed from a potential field army into a frontier police.
This policy of passive defence depended, of course, for its success upon the extent to which the frontier could be made defensible. Fortunately by this date it lay for the greater part of its length along positions of great natural strength. The Rhine, Danube, and Euphrates, when guarded by a continuous line of forts and watch-towers,[332] and patrolled by flotillas of guard-boats, formed a serious military obstacle to a raiding force, an obstacle even more dangerous to its retreat than its advance. The desert frontiers of Africa and Arabia were more easily defended, since the routes by which a hostile force could advance were limited in number and the defence could concentrate upon them, and the same of course holds true of the southern frontier of Egypt.
There were, however, districts where such natural obstacles did not exist, as in the case of the trans-Rhenane territory, which was divided between Germania Superior and Raetia, and the northern frontier of Britain, and here Hadrian had recourse to the expedient of erecting artificial barriers, which he hoped would serve the same purpose.[333] In the former case the frontier was defended by a palisade and ditch, which were later supplemented by an earth mound in the German section and replaced by a stone wall in Raetia.[334] On the British frontier, between the Tyne and Solway, the existing remains are those of a stone wall, although there are also traces of a wall of turf, which may have been an earlier work.[335] A turf wall also defended the more advanced line between the Firths of Clyde and Forth, which was occupied between 140 and 180.[336] The southern line in Britain in its most perfect form was guarded by a stone wall seventy-three miles long. This wall was between six and nine feet thick, and probably stood originally about twelve feet high. In front of it, except where the precipitous nature of the ground rendered such an additional defence unnecessary, ran a wide ∨-shaped ditch. At intervals of about every Roman mile stood a stone block-house, and between every block-house two towers. The mile-castles contained barrack accommodation for about fifty men, and reveal abundant traces of continuous occupation.[337] The garrison of about eleven thousand men lay in stone forts of the ‘cohort’ size, the majority of which are actually on the line of the wall, although a few, which probably belonged originally to an earlier system of defence, are a short distance behind it. The average interval between the forts is some six miles, so that it was easily possible for each regiment to man the adjacent towers and mile-castles and retain a considerable force at head-quarters. In addition to the troops actually stationed on the line of the wall, there were other regiments in outpost forts to the north and in the forts which guarded the three roads leading south to the legionary fortresses of Chester and York. The ends of the line were also guarded against flank attacks from the sea by forts at South Shields and on the Cumberland coast. If, then, we include all troops within three days’ march of the wall, the total force available for its defence probably exceeded twenty thousand men. Taking also into our calculations the natural strength of the position, we may safely say that this was the strongest and best guarded of all the frontier barriers.