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.