limes, in ancient Rome, originally a path that marked the boundary between plots of land. Later it came to refer to roads along which troops advanced into unfriendly territory. The word, therefore, came to mean a Roman military road, fortified with watchtowers and forts. Finally, limes acquired the sense of frontier, either natural or artificial; towers and forts tended to be concentrated along it, and the military road between them was often replaced by a continuous barrier.
The limes as a continuous barrier can best be seen in Great Britain and Germany. The Rhine and Danube rivers were adopted from ad 9 as the natural frontiers of the Roman Empire. Later in the 1st century the Romans extended their control into the Black Forest area; under the emperors Hadrian (117–138) and Antoninus Pius (138–161) a limes was established, consisting of a continuous nine-foot palisade running, in its final form, more than 300 miles across the angle between the two rivers. The palisade was later replaced by stone and earthen walls. The Alemanni broke through the limes about 260, and the Roman frontier was withdrawn to the Rhine and Danube once more. The limites in Great Britain were Hadrian’s Wall, built of stone between the Rivers Tyne and Solway and, farther north, the turf wall of Antoninus Pius between the Rivers Forth and Clyde.
The limes as a system of fortifications was employed on other frontiers during the 2nd century ad and assumed various forms according to the differing geographical and military conditions. In what is now Romania a limes of Trajanic-Hadrianic times has been traced in the Dobruja area; lines of forts to the east and west of this area, however, do not appear to have been linked by ramparts. In Anatolia a continuous barrier was neither practicable nor necessary, as the Romans controlled the roads and river crossings. In Syria, however, an elaborate limes system was established, not only to control the mobile native population and the caravan routes but also for defense against Parthian or Sāsānian attacks. The main part of this line held until the Arab conquest in the 7th century. Control of nomads was also necessary in North Africa. The network system of roads, forts, and watchtowers was adopted, but the defenses also included a continuous barrier, a ditch, and either a stone or an earthen wall. The collective remnants of these limes were designated a UNESCO World Heritage site in 2008.