A typical Roman formation relied on Its heavy infantry to wear down the enemy and its cavalry to protect its flanks. Heavy infantry were the heart of Roman military might but light infantry, missile units and cavalry were often provided by allies and subjects to help deal with different military styles from the enemy. For example, when fighting the heavy cavalry and cavalry archers of the Parthians, Romans relied on auxiliary missile units to protect themselves from the superior mobility of an enemy who often refused to engage their infantry head on.
Romans had many formations and had the training and discipline to quickly implement them on the battlefield. This is called an orb formation and is generally only used in desperate situations in which the Romans are surrounded.
This is called the testudo, or tortoise formation and it is one of the most famous Roman formations. It is very effective in protecting from enemy missile fire and although it is exhausting, Roman infantry formations were capable of moving while maintaining testudo formation. This was especially effective when approaching enemy walls as it keep the legionaries safe from their arrows and spears. However, it was far less effective against a mobile enemy as they were capable of outrunning the slow Roman advance so the legions still needed cavalry and missile units of their own.