Roman military frontiers and fortifications

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 frontiers often consisted of forts for auxiliaries, vexillations (e.g. Segedunum) or legions 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 the latter 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, like the Great Wall of China, of an attempt to construct a continuous man-made fortification along the length of an entire border, a massive undertaking.

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.

The value of the frontier 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.

[citation needed] After conquering much of the modern landmass of Great Britain, the Romans halted their northern expansion at the southern fringe of Caledonia, what is now central Scotland.

Although the border was not a continuous wall, a series of fortifications known as Gask Ridge in mid-Scotland may well be Rome's earliest fortified land frontier.

Each shore fort both protected against direct attack and also sheltered a small naval sub-fleet of vessels that could patrol the coast against pirates and raiders.

Rome advanced beyond the Euphrates for a time upon defeating their rivals, the Parthians in 116 AD, when Trajan captured Ctesiphon, and established new provinces in Assyria and Babylonia.

However the Romans attempted twice to occupy effectively the Siwa Oasis (and failed) and controlled the Nile many miles into Africa until the 1st Cataract near the modern border between Egypt and Sudan.