rare instances of circular or semicircular towers.[239] As Schultze has pointed out, the planning of towers varies with time and place, but not infrequently rounded and rectangular towers can be seen on buildings of the same date.[240] As at Zindjirli the rounded tower denotes a technical advance, though the rectangular tower is not necessarily displaced by it. The typically Roman conception of frontier defences, the fortified limes, was definitely abandoned in Europe about the year A.D. 360, but a century earlier the invasion of Gaul and Spain by the Franks had proved that the long line of strongholds was powerless to check the inrush of barbarian hordes, and in the last half of the third century the fortified town was virtually substituted for the fortified frontier. Towered walls sprang up about the cities of Roman Gaul, and the work of fortification begun by Probus was carried on by Diocletian.[241] The same process can be observed throughout the empire during the course of the third century, and almost without exception these later fortifications were strengthened by circular or semicircular towers. But if the walls of Roman cities can claim to have inherited, through Greece and the civilizations of the Aegean, the formulae of the ancient East, the fortified camp was essentially the creation of Rome herself. The stockaded earthwork, with rounded corners and lines devoid of flanking defences, determined the plan of the stone wall which replaced it in Europe and in Africa,[242] and it was not until the Romans applied their system to lands which had seen the birth and development of a science of warfare different from their own that they modified their design. The difference was fundamental. The Roman camp was intended primarily for purposes of attack. It was the camp of an army on the march, indispensable, in the eyes of commanders as wary as they were daring, to a halt that lasted no longer than a single night, but in its essence impermanent. The oriental fortress displays a contrary intention. It was defensive and abiding, a stronghold provided with few exits (since the gateway is the weakest point of a fortified position), but with high walls, heavily flanked by towers which would give the garrison every advantage against the besiegers.