As already mentioned the population was divided into three tribes, each of which provided a specific set of armed forces: 100 cavalry and ten groups of a hundred infantry. Each of these groups of 100 men was commanded by a Centurion.
Growth of the city and its population together with continuous war also influenced the structure of power within the city. At first power had been organised around families and social classes but this was BCincreasingly modified around geographical districts and to include military considerations. The council of tribes which at first saw to providing the military forces was replaced by a system articulated around 5 classes of personal wealth. The number of centuries grew to approximately 200. A council of centuries ruled over them and played an active role in the ruling of the State.
Indeed there were later times when power of the military took them as far as running the state (causing great chaos and instability). This happened at the end of the Republic with the rules of Sulla and then of Marius and again during the bad years of the Empire from the years 200-300AD.