The Roman legions were masters at siege warfare. In their siege craft they combined the greatest attributes of their civilization; organization, determination, engineering and courage.
As with all their military operations, the Romans had a set plan for taking a fortified location and they practiced it; just as they practiced all their other military drills. The Romans were so taken with realistic training that Josephus said in the 1st century AD that “Their drills were bloodless battles and their battles were bloody drills.”
When confronted with a fortified camp or city that had to be conquered, the Romans first did a reconnaissance of the enemy’s defenses. This survey looked for weak spots in the walls or undefended locations.
If the commander deemed the fortified location could not be taken directly than he would begin a siege with a standard Roman legion circumvallation. Such walls were designed to cut the enemy off from all communications and any help from the outside. Often a second circumvallation was built, this one facing outward to prevent reinforcements from reaching the besieged enemy, or an enemy attacking the Romans from the rear.
After the surrounding walls were built, the Romans placed their various artillery machines. For siege warfare the three main types of engines were the onager, ballista and catapulta. The onager was a torsion catapult capable of hurling large stones hundreds of meters. The ballista and catapulta were huge crossbow-like machines that could throw stones and also large arrows, or bolts.
The Romans rarely abandoned a siege once started. For example, they besieged the City of Carthage for nearly three years during the Third Punic War.
As the siege reached its climax, the Romans would often launch direct attacks on the walls. First they would build battering rams to knock down the city gates. The Roman battering towers were sometimes 50 feet tall or more that would let the Romans attack the tops of walls directly or shoot arrows or artillery down on defenders.
Also during assaults to protect themselves the legionaries would form their shields into a tortoise (testudo) formation with shields held over their heads and on all sides to deflect arrows, stones and other projectiles.
After a city was conquered the Romans were ruthless, killing, raping and looting. Any survivors were sold into slavery or kept to be marched thru Roman as part of a triumph (triumphus) and then executed.