How was the roman army orginized

the Romans were orginised in lines

In the Roman army the legion was divided into centuries which were meant to have 100 men, but their size could vary. From the 6th to the late 4th century they were lined up in phalanxes following the Greek model of warfare. This was a large and deep (8 files or more), compact formation of heavy infantrymen. It was designed to charge the enemy line with great impact. It was suitable for large scale pitched battles. It required great discipline to hold the files compact. The men used round shields to interlock each other. It was supported by a light infantry that covered the advance of the heavy infantry by throwing javelins. It was flanked by two wings of cavalry.
In the late 4th century the Romans introduced the Samnite manipular formation. The maniples were squares of men placed in three lines and arranged like a chessboard. The maniples in each line left a one-maniple space between them and the gap was covered by the line behind so that the withdrawing troops from in front could do so without disrupting those behind. The lines were assigned according to age and experience. The youngest and most inexperienced were in the first, the veterans in the last. If the first line’s charge failed, it withdrew behind the second, which then took over. It the second also failed, it withdrew and the third took over. This system was much more flexible and manoeuvrable and better suited for hilly terrain. It, too, was supported by a light infantry.
Gaius Marius’ reform (107 BC) created the cohort. It consisted of six centuries of 80 men each. The legion had ten cohorts (1st, 2nd, 3rd, etc.). The 1st was the most senior, the 10th the least. In the 1st century AD the 1st cohort was made of five double-strength centuries with a total of 800 men. The cohort was a larger, more compact and powerful tactical unit more suited to pitched battles, which were becoming more common. The light infantry was scrapped and replaced by the auxiliaries (allied troops).