This excerpt comes from Appian’s Roman History (Historia Romana, Book X: The Illyrian Wars, Chapter 19).
It describes the Siege of Metulum (35 BC), the capital city of the Iapydes (located in modern-day Croatia), which took place during Octavian's (the future Emperor Augustus) military campaign in Illyricum.
Octavian’s forces were attempting to capture the heavily fortified city by constructing a large siege mound to reach the top of the ramparts. In response, the Metulians engaged in fierce day-and-night sorties and launched counter-defenses, constructing a second inner wall as their outer fortification began to crumble.
The Metulians harassed the Roman legionaries using specialized siege engines (tormenta—primarily ballistae and catapults/scorpions). Remarkably, these were not native weapons, they had been captured or acquired eight years earlier (in 43 BC) following the siege of Mutina (Modena), where Decimus Brutus was besieged by Mark Antony before Octavian’s intervention.