Principes form the second line in a legion’s battle formation. Armed with heavy javelins and swords, they will engage if the Hastati were unable to break the enemy formations.
They can be recruited in the central Italian provinces Latium, Umbria, Etruria, Apulia, Liguria and Campania. These are the heartlands of the Res Publica, filled with our colonies founded during the epic struggles of the past centuries.
The principes are disciplined soldiers in the prime of their life and form the second line in a legion's battle formation. They engage if the Hastati are unable to break the enemy formation. Having large reserves of fresh troops available on the battlefield to engage in critical moments is one of the Roman army’s keys to success, besides the strict discipline.
Armed with a pilum, the hasta thrusting spear, and an Italic short sword, Principes are protected by an early version of the scutum, a bronze helmet, a bronze greave on the leading leg and a small square chestplate, called pectorale, or "heart protector." As soon as the enemy comes in range they will throw their pilum to soften his formation and then engage at close quarters.
Historically, the Principes of a legion were organised in 15 manipuli, the basic tactical unit of the Roman army. Normally the legion advanced in the famous, chessboard like, quinqunx formation. Three lines of heavy infantry with broad gaps between the single manipuli covered by the next line's displaced marching units. This practice gave the Roman army a higher maneuverability on the battlefield and made it less dependant upon the terrain than the Greek phalanx. During the battle the gaps allowed beaten or retreating units as well as fresh troops to move through the lines.
The roman infantry of the late 4th and early 3rd century BC was divided by wealth in two groups. Those with the lowest property classification for military service were fighting as light infantry, separated by fighting ability into the Leves, Rorarii and Accensi. The second, more wealthy group consisted of those fighting as heavy infantry, separated by their age and fighting ability into Hastati, Principes, and Triarii.
Every male roman had the duty to serve in the infantry for 16 years, or 20 years in the case of national emergency, until he had completed his 46th year of life. Excepting those rated high enough by the censor to be members of the ordo equester. Normally every roman must have served at least ten years in the military before he was permitted to hold any political office.