Roman military frontiers and fortifications

Roman military borders and fortifications were part of a grand strategy of territorial defense in the Roman Empire, although this is a matter of debate. 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.

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. 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.

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