The Roman legionary fortified camp

Each Roman legion had a legionary fort as its permanent base. However, when on the march, particularly in enemy territory, the legion would construct a fortified camp or castra, using only earth, turf and timber. Camp construction was the responsibility of engineering units to which specialists of many types belonged, officered by architecti (engineers), from a class of troops known as immunes who were excused from regular duties. These engineers would requisition manual labour from the soldiers at large as required. A legion could throw up a camp under enemy attack in a few hours. The names of the different types of camps apparently represent the amount of investment: tertia castra, quarta castra: "a camp of three days", "four days", etc.

Panoramic view of Vercovicium on Hadrian's Wall, now known as Housesteads Roman Fort, England

Bridges

Trajan's Bridge across the Danube, the longest bridge for over a millennium

Roman Legionaries crossing the Danube River by pontoon bridge during the Marcomannic Wars, as depicted in relief on the column of Emperor Marcus Aurelius (r. 161-180 AD) in Rome, Italy

The engineers built bridges from timber and stone. Some Roman stone bridges survive. Stone bridges were made possible by the innovative use of keystone arches. One notable example was Julius Caesar's Bridge over the Rhine River. This bridge was completed in only ten days and is conservatively estimated to have been more than 100 m (300 feet) long.^[1][2] The construction was deliberately over-engineered for Caesar's stated purpose of impressing the Germanic tribes,^[3] who had little experience of engineering, and to emphasise that Rome could travel wherever she wished. Caesar was able to cross over the completed bridge and explore the area uncontested, before crossing back over the subsequently dismantled bridge. Caesar related in his War in Gaul that he "sent messengers to the Sugambri to demand the surrender of those who had made war on me and on Gaul, they replied that the Rhine was the limit of Roman power". The bridge was intended to show otherwise.

Siege machines

Roman carroballista on Trajan's Column, early 2nd century AD

Although most Roman siege engines were adaptations from earlier Greek designs, the Romans were adept at engineering them swiftly and efficiently, as well as innovating variations such as the repeating ballista. The 1st century BC army engineer Vitruvius describes in detail many of the Roman siege machines in his manuscript De architectura.

Roads

When invading enemy territories, the Roman army would often construct roads as it went, to allow swift reinforcement and resupply, or for easy retreat if necessary. Roman road-making skills were such that some survive. Michael Grant credits the Roman building of the Via Appia with winning them the Second Samnite War.^[4]

Civilian engineering by military troops

Greco-Roman Pentaspastos ("Five-pulley-crane"), a medium-sized variant (ca. 450 kg load)

The Roman army built projects for civilian use. When soldiers were not engaged in military campaigns, the legions had little to do, while costing the Roman state large sums of money. Thus, involving soldiers in building works, kept them well accustomed to hard physical labour and out of mischief, since it was believed that idle armies were a potential source of mutiny.

Soldiers built many roads across the Empire. Soldiers were also put to use in the construction of town walls, the digging of canals, drainage projects, aqueducts, harbours, and even in the cultivation of vineyards.

Mining operations

Soldiers became skilled in conducting mining operations such as building aqueducts needed for prospecting for metal veins, in activities such as hydraulic mining, and building reservoirs to hold water at the minehead.