*The fortification of the limites.* Although Claudius and Vespasian discarded the maxims of Augustus in favor of an aggressive border policy they adhered to his system for protecting their new acquisitions in Britain and the Agri Decumates. However, these conquests and that of the Wetterau region by Domitian pushed the frontier beyond the line of natural defences and led to the attempt to construct an artificial barrier as a substitute. It was Domitian who took the initial step in this direction by fortifying the _limites_ between the Rhine and Main, and the Main and the Neckar, with a chain of small earthen forts connected by a line of wooden watchtowers. To the rear of this advanced line there were placed larger stone forts, each garrisoned by a corps of auxiliaries, and connected by roads to the posts on the border. While the auxiliary troops were thus distributed along the frontiers in small detachments, the larger legionary cantonments were broken up, and after 89 A. D. no camp regularly contained more than a single legion. Trajan, who also waged his frontier wars offensively, merely improved the system of communication between the border provinces by building military highways along the line of the frontier from the Rhine to the Black Sea, in Arabia, and in Africa. In the matter of frontier defence, as in so many other spheres, a new epoch begins with Hadrian. He reverted abruptly to the defensive policy of Augustus and began to fortify the _limites_ on a more elaborate scale. The frontier between the Rhine and the Danube was protected by an unbroken line of ditch and palisade, in which stone forts, each large enough for an auxiliary cohort, took the place of the earthen forts of Domitian. At the same time the _limes_ was shortened and straightened, and the secondary line of forts abandoned. In Britain a wall of turf was constructed from the Tyne to the Solway, and in the Dobrudja a similar wall linked the Danube to the Black Sea. The eastern frontier of Dacia was likewise defended by a line of fortifications. Here, as on the other borders, the Roman sphere of influence, and even of military occupation, extended beyond the fortified _limes_.