Roman Military Architecture and Defences

The Roman army, known in Latin as exercitus Romanus, served the Roman state for over two millennia. It existed through the Roman Kingdom, Republic, and Empire, spanning from 753 BC to AD 1453. This extensive timeline includes the Western Roman Empire, which collapsed around AD 476, and the Eastern Roman Empire, which fell in AD 1453. The army underwent numerous changes in size, organization, and tactics while maintaining core traditions.

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Early Roman Army (c. 550 – c. 300 BC)

Before 550 BC, Rome did not have a unified army. Instead, it relied on clan-based war bands that formed during external threats. Around 550 BC, under King Servius Tullius, a universal levy of eligible male citizens was established. This change coincided with the introduction of heavy armor for infantry. Initially, the Roman army was small but effective, developing influential battle strategies.

The early army was based on compulsory levies from adult male citizens at the start of each campaigning season. There were no standing forces. During the regal period, the standard levy was likely around 9,000 men, including heavily armed infantry, light infantry, and light cavalry. After the monarchy ended, the army was divided between two annually elected praetors, each commanding one legion of 4,500 men.

In battle, the hoplite element likely fought in a phalanx formation, although most conflicts were small-scale skirmishes. The basic tactical unit was the *centuria*, consisting of 100 men. Clan-based forces continued to exist until around 450 BC, operating under the authority of the praetors.

In 493 BC, shortly after the Roman Republic’s establishment, Rome formed a military alliance with other Latin city-states through the *Foedus Cassianum*. This treaty required each party to provide equal forces for campaigns under unified command, remaining in effect until 358 BC.

Roman Army of the Mid-Republic (c. 300 – 107 BC)

The mid-Republic army, known as the Polybian army, featured a manipular organization. This structure replaced the phalanx with three lines of small units called maniples, each containing 120 men. This change likely occurred around 300 BC during the Samnite Wars. Each legion was accompanied by a non-citizen formation called the ala, recruited from Rome’s Italian allies.

During the Second Punic War, non-Italian mercenaries were added to the army, including Numidian cavalry and Cretan archers. The Republican army did not maintain standing forces but levied troops through conscription for each campaign. The standard levy increased during the Samnite Wars to four legions, totaling around 18,000 Roman troops.

Service in the legions was limited to property-owning Roman citizens, typically aged 16 to 46. Senior officers were elected annually, and only members of the equestrian class could serve as senior officers. The army’s cavalry evolved from light, unarmored horses to a heavy force with metal armor.

The Polybian army’s operations can be divided into three phases: the struggle for hegemony over Italy, the conflict with Carthage, and the struggle against Hellenistic monarchies. The size of the levy during the earlier phase was around 40,000 men.

Imperial Roman Army (30 BC – AD 284)

Under Emperor Augustus, the legions transformed into all-volunteer units of long-term professionals. The average service term increased to 25 years, with conscription only in emergencies. By the late 1st century, the size of a legion’s First Cohort was doubled, raising the total personnel to about 5,500.

Augustus also established the auxilia, a corps of non-citizen troops that provided cavalry and specialist infantry. The auxilia were organized into units called cohortes and alae. By around 80 AD, most auxiliary regiments were volunteer corps. Upon completing their service, auxiliaries were granted Roman citizenship.

By the end of Augustus’ reign, the imperial army numbered approximately 250,000 men, with numbers peaking at around 450,000 by 211 AD. The military chain of command was uniform across the Empire, with legion commanders reporting to provincial governors, who reported to the emperor.

Legionaries were relatively well-paid compared to common laborers. They received bonuses on special occasions and a generous discharge bonus upon completing their service. Junior officers and centurions earned significantly more than basic pay, while senior officers received high salaries, often multiples of basic pay.

Late Roman Army (284–641)

The Late Roman army refers to the military forces from Diocletian’s accession in 284 until the Empire’s division in 395. The army underwent significant changes due to the chaotic 3rd century. Unlike the earlier army, the 4th-century army relied heavily on conscription and was less well-paid.

Estimates of the late army’s size vary. Some scholars suggest it was larger than the Principate army, while others argue it was similar in size. The main structural change was the establishment of large armies accompanying emperors, which were based away from frontiers. The legions were divided into smaller units similar to auxiliary regiments.

The role of cavalry did not significantly change compared to the earlier army. The infantry maintained its reputation for excellence, while cavalry faced criticism for incompetence in major battles. The 3rd and 4th centuries saw upgrades to border forts and the construction of new, more defensible structures.