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Roman military borders and fortifications were part of a grand strategy of territorial defense in the Roman Empire. By the early 2nd century, the Roman Empire had reached the peak of its territorial expansion and rather than constantly expanding their borders as earlier in the Empire and Republic, the Romans solidified their position by fortifying their strategic position with a series of fortifications and established lines of defense. Historian Adrian Goldsworthy argues that the Romans had reached the natural limits which their military traditions afforded them conquest over and that beyond the borders of the early-to-mid Empire lay peoples whose military traditions made them militarily unconquerable, despite many Roman battle victories. In particular, Goldsworthy argues that the cavalry-based warfare of the Parthians, Sarmatians and Persians presented a major challenge to the expansion of Rome's infantry-based armies. ==Nature of the fortifications== The borders of the Roman Empire, which fluctuated throughout the empire's history, were a combination of natural frontiers (the Rhine and Danube rivers to the north and east, the Atlantic to the west, and deserts to the south) and man-made fortifications which separated the lands of the empire from the "barbarian" lands beyond. Individual fortifications had been constructed by the Roman military from as early as the building of Rome's first city walls in the 6th or 7th century BC. However, systematic construction of fortifications around the periphery of the empire on a strategic scale began around 40 AD under Emperor Caligula. However, it was under Hadrian's rule, which began in 117, that the Roman frontier was systematically fortified. He spent half of his 21-year reign touring the empire and advocating for the construction of forts, towers, and walls all across the edges of the empire.〔Curry, Andrew. "Roman Frontiers." ''National Geographic'' Sept 2012: 106-127. Print〕 The coherent construction of these fortifications on a strategic scale (''i.e.'' to protect the empire as a whole rather than fortifying individual settlements) are known as the ''limes'', and continued until around 270. The ''limes'' consisted of fortresses for legions or vexillations (e.g. Segedunum) as well as a system of roads for the rapid transit of troops and, in some places, extensive walls. Perhaps the most famous example of these is Hadrian's Wall in Great Britain, which was built across the entire width of the island to protect from attack from tribes located in modern-day Scotland. The so-called Limes Britannicus is perhaps the best example of the ultimate ''limes'' - like the Great Wall of China, it was an attempt to construct a continuous man-made fortification along the length of an entire border, a massive undertaking. However, it is not correct to interpret other ''limes'' in the same way or to view the ''limes'' as an impenetrable barrier. Other limes would not have had a continuous man-made fortification for the entirety of their length. In places, a river, desert or natural outcropping of rock could provide the same effect for zero outlay. Also, fortifications as impressive as Hadrian's Wall were not unbreachable: with milecastles some distance apart and patrols infrequent, small enemy forces would have been able to penetrate the defenses easily for small-scale raiding. However, a raiding party would be forced to fight its way through one of the well-defended gates, abandon its loot, such as cattle, thus negating the whole purpose of the raid or be trapped against the wall by the responding legions. Additionally, a large army would have been able to force a crossing of the ''limes'' using siege equipment. The value of the limes lay not in its absolute impenetrability but, as S. Thomas Parker argues, in its hindrance to the enemy: granting a delay or warning that could be used to summon concentrated Roman forces to the site. The ''limes'' are therefore perhaps better seen as an instrument allowing a greater economy of force in defense of a border than otherwise would be necessary to provide the same level of defense. After 270, the maintenance of an impenetrable solid frontier was abandoned by Constantine I in favor of a policy, whether deliberate or forced by circumstance, of "defense in depth". This called for the maintenance of a softer, deeper perimeter area of defense, with concentrated hard points throughout its depth. The idea was that any invading force of a sufficient size could penetrate the initial perimeter but in doing so with any element of surprise or rapid movement would be forced to leave several defended hard points (fortresses) to its rear, hampering its lines of supply and communications, and threatening surrounding of the force. In the very late Empire the frontiers became even more elastic, with little effort expended in maintaining frontier defense. Instead, armies were concentrated near the heart of the empire, and enemies allowed to penetrate in cases as far inwards as the Italian peninsula before being met in battle. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Roman military frontiers and fortifications」の詳細全文を読む スポンサード リンク
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