This sketch of the methods of defence employed applies more particularly to the German and Raetian frontiers. In Britain, more particularly on the southern line, it is probable that a more serious defence was intended. In the first place, the massive stone wall, on which the defenders could stand, was obviously stronger than anything on the trans-Rhenane section.[344] Secondly, we have noted that the garrison was stronger than in Germany, and could be more easily reinforced. Moreover, even after the final abandonment of Scotland, forts were still held in front of the southern line. Netherby on the Esk, and Habitancium (Risingham) and Bremenium (High Rochester) on Dere Street, were occupied by _cohortes miliariae equitatae_ well into the third century, and at the last two forts we find a _numerus exploratorum_ attached to the regular auxilia.[345] These strong outposts would have been able to check or harass the enemy’s advance and give warning to the garrison of the wall of any impending attack. All these suppositions, however, both as regards Germany and Britain, are based upon the assumption that a raid would be the sole subject of the attacking force, and that it would not be too numerous to be dealt with by the garrisons of three or four _castella_. To a more serious invasion the resistance offered was much less effective. The legions, it is true, still remained in reserve, but they formed the only concentrated force at the disposal of the defending general, for the majority of the auxilia, scattered as they were along the entire length of the frontier, could not be quickly concentrated, and a provincial garrison can rarely have taken the field at anything like its full strength. The system also created serious difficulties when it became necessary to send troops from one province to the aid of another. Three regiments, for example, could not easily be sent from Germany to Pannonia, because each of them constituted an essential link in the chain of frontier defence. It became the practice, therefore, to form out of detachments drawn from several regiments a composite _vexillatio_ in which efficiency must have been greatly diminished by lack of _esprit de corps_. A cavalry _vexillatio_ of this type commanded by a certain Lollianus, probably during the Parthian war of Trajan, was drawn from no less than five _alae_ and fourteen _cohortes equitatae_.[346]