This is ONLY an overview. The Roman Army has an extensive history which only years of study can broach.
1. The Infantry, pedites. This consisted exclusively of Roman citizens, who could be called out to service between the ages of seventeen and forty-six. Many, however, were volunteers, who enlisted for twenty years. In addition to their pay - about $45 a year - they shared in the booty that was taken, and often received special gifts. At the end of their term of enlistment they were retired with a lump sum of money or an allotment of land.
The Legion, legio. The full strength of a legion was 6000 men, but losses in battle, sickness, and desertion reduced the number. In the Gallic war Caesar's legions contained an average of about 3600 men. The legion was divided as follows:
1 century (centuria) = 60 men.
2 centuries = 1 maniple (manipulus), 120 men.
3 maniples = 1 cohort (cohors), 360 men.
10 cohorts = 1 legion, 3600 men.
2. The Cavalry, equites. Caesar's cavalry was composed of foreigners - Gauls, Germans, and Spaniards, of whom the Germans were by far the best. The cavalry force, attached to each legion, was divided as follows:
1 squad (decuria) = 10 men.
3 squads = 1 troop (turma), 30 men.
10 troops = 1 squadron (ala), commanded by a cavalry prefect (praefectus equitum).
3. The Auxiliaries, auxilia, commanded by Roman officers, consisted of:
Light-armed foot soldiers (levis armaturae pedites), drawn from allied or subjectstates. In battle these were generally posted on the wings to make a show of force. At other times they were used chiefly for raiding and foraging.
Slingers, funditores, from the Balearic Islands.
Bowmen, sagittarii, from Crete and Numidia.
Camp-servants, calones, slaves who performed menial services about teh camp or were the body servants of the officers.
Muleteers, muliones, who drove the pack animals and had charge of the heavy baggage.
Traders, mercatores, who were permitted to accompany the army, but - except in time of danger - were not allowed to have quarters in the camp. They purchased booty from the soldiers, and in turn sold them food and clothing.
The Commander-in-chief, dux, until he had won his first important victory, after which he received the title of imperator
The Lieutenant generals, legati, who were appointed by the Roman senate from the senatorial class. They were the commander's staff officers. They were not in charge of any special legion, but in battle were assigned by the commander to whatever legion he saw fit.
The Quartermaster general, quaestor, whose duty it was to pay the men and to pruchase the supplies for the army. If he was a capable officer, he might be put in command of a legion in battle.
The Military tribunes, tribuni militum, six to each legion. They were usually young men of equestrian families, without any military experience, who served only a short time and then returned to Rome to take up a political career. Their duties were of minor importance, such as the command of small detachments, the providing of supplies, and the levying or discharge of soldiers.
The Prefects, praefecti, who commanded the auxiliaries and the cavalry squadrons.
The Centurions, centuriones, non-commissioned officers of plebeian origin, who were promoted from the ranks for their bravery and efficient service. There were two centurions for each maniple, six for each cohort, and sixty for each legion. The ranking centurion was the first centurion of the first cohort (primipilus).
the Decurions, decuriones, who were cavalry leaders of the squads (decuriae).
The Evocati, veteran volunteers, who had reenlisted after serving for twenty years.
Signiferi and Aquiliferi who were standard bearers
The legionary soldier wore:
tunica, a short-sleeved tunic, reaching nearly to the knees.
sagum, a thick woolen cloak.
caligae, heavy leather sandals, fastened to the foot by many straps. Instead of the sagum the commander wore a distinctive scarlet cloak, paludamentum
galea, a helmet of leather and bronze, adorned with a crest. While marching the helmet was hung by a cord from the neck.
lorica, a leather cuirass or coat-of-mail, strengthened with metal strips.
scutum, an oblong wooden shield, overlaid with linen or leather, with a metal rim. It was about 4 feet long and 2½ wide. In the middle on the outside was a metal boss, umbo. The shield was variously ornamented, often with a winged thunderbolt. A leather covering, tegimentum, encased it, except in battle. It was worn on the left arm.
pilum, a heavy pike or javelin, weighing about ten pounds. It consisted of a wooden shaft 4 feet long, into which was fitted and iron shaft 2 feet long. The latter was made of soft iron, but its barbed point was hardened. The pike could be hurled about 75 feet.
gladius, a sword, about two feet long, straight, pointed, and two edged, used rather for quick thrusting than for slashing. It was kept in a scabbard, vagina, which was suspended on the soldier's right side by a strap, balteus, passing over the left shoulder.
aquila, the standard of the legion, a silver eagle on a wooden staff carried by the aquilifer.
signa, the standards of the maniples (born by hte signiferi), of various designs. For example one type had an open hand at the top follows by two metal disks, with an oblong metal plate below, on which were inscribed the letters indicating the place of the maniple. The standard bearers wore wolf or bear skin over their head and shoulders.
vexillum, a small rectangular, bright-colored banner, the standard of the cavalry and the infantry auxiliaries. A large red vexillum, displayed over the commander's tent, was the signal to make ready for battle.
The Musical Instruments
tuba, a long straight trumpet, with deep tone.
cornu, a curved horn, with shriller note.
bucina, a bugle, used chiefly to sound orders within the camp.
In battle the trumpeters, tubicines, first sounded the general's orders, which the horn-blowers, cornicines, in turn sounded to the maniples to which they were attacked.
Caesar's army was highly organized
and each soldier had his appointed
duties. This man sounded his trumpet
at regular intervals during the day,
as the watches changed.
The propelling power of Roman engines of war was secured by windlassing strong hair ropes and then suddenly releasing them by means of a trigger. They were called tormenta, from torqueo, twist because the ropes were twisted.
catapulta, the catapult, which shot large arrows or darts horizontally.
scorpio, the scorpion, a small catapult.
ballista, the ballista, which hurled huge stones at an angle of about 45 degrees.
The Army on the March
the Roman army marched in three divisions:
primum agmen, the vanguard, composed of cavalry, and perhaps some light-armed troops.
agmen, the main column, each legion being followed by its own baggage train. But if there was a danger of sudden attack, the entire baggage was placed behind the legions that formed the agmen.
novissimum agmen, the rear-guard, as a rule consisting of the more recently enlisted legions (hence called "the newest column").
A regular day's march lasting six or seven hours began at sunrise, but in emergencies at an earlier hour, sometimes even at midnight. The average distance marched was 15 or 16 miles, but Caesar's army often covered 25 or 30 miles on a forced march, iter magnum. On the march each legionary carried a pack containing his blanket, rations, cooking utensils, etc. These packs, called sarcinae, were fastened on a forked pole and carried over the left shoulder.
The Camp (Castra)
At the close of each day's march the Roman army regularly constructed a fortified camp. A crops of engineers was sent ahead of the army to select a suitable site and lay it out. Usually it was rectangular, its dimensions varying according to the size of the army. It had four gates.
The camp was fortified by:
fossa, a trench with sloping sides, 10-12 feet wide at the
top, and 7-9 feet deep.
vallum, a rampart, constructed from the earth and stones that were dug out of the trench. It was 5-6 feet high and 6-8 feet wide on top. On the outer edge of the rampart was a breastwork of stakes, valli, affording protection to soldiers posted on top of the wall.
The interior of the camp was laid out in a systematic way. The main street, via principalis, ran the full length of the camp, being crossed at right angles by the via praetoria. A number of passage-ways, viae, ran across the camp both lengthwise and widthwise. The general's tent, praetorium, was near the middle, with the officers' tents close by. The soldiers' tents, each accomodating ten men, were nearer the front.
The Army in Battle
As a rule Caesar's legionaries were drawn up for battlei n three lines, acies triplex, as follows:
The three cohorts in the second line were so placed that they could move up in the spaces between the cohorts, and relieve them if they became exhausted. The third line was held in reserve, to enter the battle in an emergency.
Operations Against Walled Towns
Towns might be captured by storming the walls or by a regular siege.
In storming, oppugnatio, the soldiers first filled the trench or moat with earth and bundles of brush, crates, and then pressed forward to break down the gates with a battering ram, aries, and mount the walls with ladders, scalae. Gruops of soldiers advanced under cover of their shields overlapping above their heads. This formation was named tetsudo, tortoise, because they were protected by it like a tortoise in its shell.
In a siege, obsidio, the entire town, if possible, was invested with a line of works. Then at a distance of about 400 feet from the weakest point in the wall the soldiers began an agger, an inclined roadway - made of logs and earth - 40 or 50 feet wide, which was eventually to reach the wall at a level with the top. When it was extended nearer the enemy's wall, the soldiers at work upon it were shielded from missiles by movable screens, plutei, or by sheds, vineae, mounted on rollers.
Movable towers, turres ambulatoriae, were built at a safe distance from the wall, and at the proper time were pushed forward on rollers to take part in storming the walls.
These notes come from a college Overview, Scudder, 2Y Latin.