No system of connected operations can be traced between prehistoric forts. Each of these was probably an isolated stronghold. Roman stations, on the other hand, were military posts manned by detachments of one army, and connected by strategic roads. This is seen very clearly in the case of the great wall already mentioned. The wall can be traced for about 73 miles, from Wallsend (Segedunum) on the Tyne to Bowness on the Solway. It is built in the usual Roman method, with a core of cemented rubble between ashlar facings. Its breadth varies between 7 and 9½ feet: the height appears to have been originally from 16 to 18 feet.[6] Its northern face is defended by a ditch, marked _a_ in the section below; but, as it follows the highest ground in its course, and runs for some distance along the edge of basaltic cliffs which dip northward, the ditch at these points becomes unnecessary and is dispensed with. There were twenty-three stations in its length, each garrisoned by a cohort of infantry or squadron of cavalry, chosen from the Roman auxiliary troops of Gauls, Spaniards, Moors, &c.[7] These were connected by a paved military road (_c_) along the south side of the wall. At distances of a Roman mile from one another were placed rectangular forts, now known as mile-castles, built against the wall upon its south side; and the interval between each of these was strengthened further by turrets, apparently two in number, which also projected southward, but encroached slightly upon the thickness of the wall. The south side of the military road was defended by an earthen _vallum_, marked _d_,[8] the course of which is not directly parallel to the wall, but is in places as much as half a mile distant, keeping to lower ground where the wall prefers the summit of the basalt ridge. This bank has a ditch (_e_) on its southern side, divided from it by a level space or berm: the ditch has a southern parapet (_f_), and another bank beyond (_g_). In certain places the ditch has also a northern parapet; but the general arrangement of the earthen ramparts is as described. Of the controversy as to the relative date of the wall and its flanking earthworks nothing need be said here. Its military purpose is abundantly clear. It provided not only a strong means of defence against the attacks of the northern tribes, but a base of operations for offensive warfare. Each of the stations and mile-castles has a northern gateway in addition to its other entrances; and two of the Roman roads which met the wall at intervals from the south passed through it on their way to the Scottish border. [Illustration: Section of Roman Wall and _Vallum_.]