Roman fortifications at the borders were part of the great defense strategy of the Roman Empire. At the beginning of the second century CE, Rome reached the height of its conquests and decided to strengthen its gains by building fortifications at key border locations. As historian Adrian Goldsworthy underscores the Romans have gained as much as possible in the state, considering their reliance on heavy infantry. When the Parthians, Sarmatians and Persians appeared in the political arena, basing their military power on the cavalry, the Romans had to make a capitulation.
The boundaries of the Roman empire, which have changed over the centuries, have been bordered by natural boundaries (Danube and Rhine in the north and east, Ocean in the west, and deserts in the south) and fortifications created by the Romans to separate the civilized world from the barbarians.
Roman fortifications were built already in the seventh or sixth centuries BCE when the capital was surrounded by the first walls. However, the first systematic work on defensive constructions, placed at the borders, was carried out only in the days of the emperor Caligula around the year 40 CE.
The real extension of the Roman limes occurred under the reign of Trajan in 117 CE. The emperor spent half of his 21-year rule by traveling around the empire and supervising the construction of forts, walls, towers and defense gates at the state borders. The whole initiative (strategy) was to defend the state, not individual regions or human groups. The expansion of limes was carried out up to 270 years ago.
Limes consisted of forts intended for legions and vexillationes (separated from the mother legion), network of walls and fortifications and a road system designed to accelerate the movement of troops and supplies. The biggest of this type was certainly the Hadrian’s Wall built in Britain, which was also referred to as Limes Britannicus. It’s aim was to defend Roman territories in Britain, through man-made defensive fortifications.
After 270 years CE, gradually, the idea of defending the borders on limes was abandoned, so that during the reign of Constantine I due to the continuous pressure of barbarians, state focused on defense in the depths of territories. At the end of the existence of the Western Roman Empire, the borders were so flexible that Romans did not even try to defend them from the enemy’s invasion. Mainly, the forces were kept in the most important places of the state, often very close to the capital. This situation enabled the hordes of barbarians to plunder many Roman cities and even prowl the Apennine peninsula.