Roman Army Formations - Holding the Lines
The Roman Army Formations were varied depending on the situation. Under ordinary circumstances legions were formed in a triple line. When the Roman force was far outnumbered by the enemy, the legionary soldiers were arranged in a double or even single line. The first line would have 2400 men, standing in 4 cohorts, 10 ranks deep. The second and third lines would each have 1800 men. The first rank of the legion would extend about 720 feet whereas a triple line of battle might extend a mile or a mile and a half long.
Roman Army Formations - Strategy and Tactics
The Roman soldiers in each battle line stood about three feet apart each way. As the first line went into action the second followed closely behind; as the men of the first fell or withdrew exhausted, those of the second pressed forward and took their places; in case of need the third line advanced and in like manner relieved the combined first and second. Sometimes there would be a wedge-shaped column or soldiers would shelter under a tortoise cover of shield called a testudo. The cavalry were used by skirmishing to prevent enemy flank movements, as diversions to allow for re-positioning and were used to prevent the escape of enemies who tried to flee.
Roman Army Formations - Order of Battle
The commanders of the Roman army would place their infantry in the middle with auxiliaries on the right and left sides. There would be a large reserve of infantry men behind the main force. Units of cavalry backed the auxiliaries on the right and left sides and formed the rear flanks. Auxiliaries (from Latin: auxilia = "supports") formed the standing non-citizen corps of the Roman army of the Principate (30 BC–284 AD), alongside the citizen legions. Auxiliary recruits were mostly volunteers, not conscripts providing specialist support to the legions, an example of this would be the Syrian archers. The Commanders would observe and issue orders from the rear but would join the troops as the battle developed.
Roman Army Strategy, Tactics and Fighting Formations by Vegetius Renatus in De Re Militari
Publius Flavius Vegetius Renatus was a writer of the Later Roman Empire and the author of De Re Militari (Latin "Concerning Military Matters"). De Re Militari is a treatise of Roman warfare and military principles and the following excerpt describes the training, strategy, tactics and fighting formations used in the Roman Army during his era.
"No part of drill is more essential in action than for soldiers to keep their ranks with the greatest exactness, without opening or closing too much. Troops too much crowded can never fight as they ought, and only embarrass one another. If their order is too open and loose, they give the enemy an opportunity of penetrating. Whenever this happens and they are attacked in the rear, universal disorder and confusion are inevitable. Recruits should therefore be constantly in the field, drawn up by the roll and formed at first into a single rank. They should learn to dress in a straight line and to keep an equal and just distance between man and man. They must then be ordered to double the rank, which they must perform very quickly, and instantly cover their file leaders. In the next place, they are to double again and form four deep. And then the triangle or, as it is commonly called, the wedge, a disposition found very serviceable in action. They must be taught to form the circle or orb; for well-disciplined troops, after being broken by the enemy, have thrown themselves into this position and have thereby prevented the total rout of the army. These evolutions, often practised in the field of exercise, will be found easy in execution on actual service".