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_.

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 sa