Antonius Pius followed Hadrian’s example and ran an earthen rampart with forts at intervals from the Forth to the Clyde in northern Britain. This line of defence was abandoned by Septimius Severus, who rebuilt Hadrian’s rampart in the form of a stone wall with small forts at intervals of a mile and intervening watch towers. In addition seventeen larger forts were constructed along the line of the wall. The _limes_ in Germany was strengthened by the addition of a ditch and earthen wall behind Hadrian’s palisade, but along the so-called Raetian _limes_, between the Danube and the Main, another stone wall, 110 miles long, took the place of the earlier defences. A similar change was made in the fortifications of the Dobrudja. However, this system was not followed out in the East or in Africa, where the _limes_ was guarded merely by a chain of blockhouses. *The consequences of permanent fortifications.* The result of the construction of permanent fortifications along the frontier was the complete immobilization of the auxiliary corps. Stationed continuously as they were for the most part in the same sectors from early in the second century, and recruited, in increasing proportion, from among the children of the camps, it only required the granting to them of frontier lands by Severus Alexander, upon condition of their defending them, to complete their transformation into a border militia (_limitanei_). At the same time the scattering of the legions along the line of the frontiers made the assembling of any adequate mobile force a matter of considerable time. And the fortifications themselves, while useful in checking predatory raids by isolated bands and in regulating intercourse across the frontiers, proved incapable of preventing the invasion of larger forces. Consequently, when in the third century the barbarians broke through the _limites_ they found no forces capable of checking them until they had penetrated deeply into the heart of the provinces. The chaos which followed the death of Severus Alexander was the result of a military policy which left the richest and most highly civilized parts of the empire without any means of self-defence; created a huge professional army the rank and file of which had come to lose all contact with the ungarrisoned provinces, all interest in the maintenance of an orderly government and all respect for civil authority; and at the same time rendered the army itself incapable of performing the task for which it was organized.