The main battle tactics relied on the individual units involved working as a team, with every unit knowing exactly what part they played. Each was extremely well trained and disciplined with a set battle plan. They were total killing machines, each armed with a sword (gladius), two javelins (pilum) and a shield (scuta)
When confronting an enemy in battle, the structure of the roman advance was to have the legion's soldiers in a line marching forward with their shields raised facing the enemy. They would be the first to do battle against the enemy soldiers. At each end on the flanks would be the Ausilia. Their purpose was to protect the legion's soldiers from any attacks from the side. Behind these two would be the cavalry. Soldiers on horseback with longer swords ready for action once the infantry had played their part.
The main plan of attack would be for the Romans to position themselves at the top of a rise, thereby forcing the enemy to run up the slope towards them.
When they were about 30 metres apart, a trumpet would sound in the Roman ranks and this would be the signal for the legionaries to release their two javelins in quick succession. The visual effect of several thousand javelins in the air at one time was terrifying to the enemy, causing them to stop and crouch with their shields above head height to protect themselves from these missiles. This would be difficult as the enemy's ranks further back would still be pushing forward, so causing a concertina effect of bodies. When the javelins landed, the sharp points would pierce the shields and the softer shaft would fold under the impact. This would render the shields useless as they could not be carried with such an awkward tangle...