When the Roman army invaded new lands it often found walled towns and fortresses that had to be conquered. The army attacked with special weapons, towers and ladders.
The ballista was a powerful ancient crossbow. It fired iron tipped bolts towards the enemy positions. The bolts whistled through the air at some 50 metres per second, and carried a terrifying punch; they would go through armour, and cause instant death.
The most powerful artillery piece in the Roman arsenal was the sling equipped Onager. It could fire rocks of up to 150 lbs (70 kgs) to be used to smash through walls and fortifications. Each rock was loaded into a sling on the end of an arm.