Roman armies were called legions, consisting of about 5000 soldiers and officers, most of whom were Roman citizens. Each legion was divided into cohorts, the basic functional unit of the Roman military. A cohort had 400-600 soldiers further divided into six centuries of 80-100 men each. Each century was commanded by a centurion, with the most senior centurion also commanding the cohort. Units of cavalry and light infantry supported the main force of heavy infantry. 

Each soldier carried a long spear (hasta, pilum) and a large curved rectangular shield (scutum) over a meter tall. A short sword (gladius) was carried as a secondary weapon. Armor consisted of overlapping strips of metal over the torso (lorica segmenta), a helmet with cheek plates (galea), and greaves over the shins.

The Romans fought in a phalanx formation, where multiple rows of soldiers moved forward as a single unit, constantly pushing on and crushing the opposing force. Overlapping shields provided protection from enemy weapons, while spears held overhead and thrust downward made short work of the enemies themselves. Roman legions were highly disciplined and trained, and they were considered the deadliest military force of their time.