The Greek ballista was a siege weapon. All components that were not made of wood were transported in the baggage train. It would be assembled with local wood, if necessary. Some were positioned inside large, armoured, mobile siege towers or even on the edge of a battlefield. For all of the tactical advantages offered, it was only under Philip II of Macedon, and even more so under his son Alexander, that the ballista began to develop and gain recognition as both a siege engine and field artillery. Historical accounts, for instance, cited that Philip II employed a group of engineers within his army to design and build catapults for his military campaigns. There is even a claim that it was Philip II - with his team of engineers - who invented the ballista after improving Dionysius's device, which was merely an oversized slingshot. It was further perfected by Alexander, whose own team of engineers introduced innovations such as the idea of using springs made from tightly strung coils of rope instead of a bow to achieve more energy and power when throwing projectiles. Polybius reported about the usage of smaller, more portable ballistae, called scorpions, during the Second Punic War.