There is no such thing as a “medieval ballista”. Same way there is no “WWII trebuchet”.
“Ballista” is a Latin corruption of the Greek “ballistra” based on the greek word for “throw”. Basically its Greek for throwing machine and refers to ancient torions weapons. odly enough no one used ancient weapons in the middle ages, they used medieval weapons. the only torsion machine used during the middle ages was the springald that would have been laughed and jeered off the field in any ancient battle.
Sugest you pre-order a copy of Slings and Arrows – the Evolution of Pre-Gunpowder Artillery today.
It’s such a job correcting 600 years of missinformation.