Under the principate the career of a centurion played an important role in military, and social aspects of Roman society. The centurions formed the backbone of the Roman army and provided a limited avenue of social mobility. According to modern scholars L. Keppie and R. Alston, the centurionate were the highest positions an ordinary soldier can aspire. Epigraphic evidence also suggests that ambitious equestrians used the centurionate as a stepping-stone to imperial service; some centurions received considerable status and wealth upon retirement.
According to L. Keppie the legion of the principate numbered between 5000-6000 men divided into 10 cohorts, individual fighting units which included 6 centuries of 80 men each commanded by a centurion. There were 59 centurions in total with a strict hierarchy within the ranks. The most prestigious been the primi ordines, 5 centurions of the first cohort, who commanded the most experienced troops, each with 160 men.
The primus pilus, centurion of the 1st century of the 1st cohort was the pinnacle of success for an ordinary soldier. It is difficult to draw an exact parallel equivalent to modern ranks but they should not be thought of as sergeants rather as commanders or middle ranking officers. As the high-ranking officers, legates and tribunes were more likely to be political appointees based on loyalty and status rather than military experience. Therefore it was the centurions that were relied upon to keep the army running.
Centurions were professional career soldiers who were in charge of the daily lives of their men on and off the battlefield. From Keppie, "They provided continuity of standards and traditions. Equally however they must have been bastions of conservatism, averse to innovation & change." Centurions issued various camp duties, supervised training and enforced discipline among his men. On the battlefield centurions were the drillmasters...