By the medieval period, ballistae had mostly been replaced by more modern designs. The remaining ones were usually in defensive emplacements on castles and the like, so I suppose you would just set them up to aim along the approaches to your castle. They are typically built in locations that are difficult to approach specifically to make them easier to defend, after all, so you can probably be pretty confident of which direction the enemy will be in
Look we just need to get the enemy to walk in front of it. I don’t see the problem.
By the medieval period, ballistae had mostly been replaced by more modern designs. The remaining ones were usually in defensive emplacements on castles and the like, so I suppose you would just set them up to aim along the approaches to your castle. They are typically built in locations that are difficult to approach specifically to make them easier to defend, after all, so you can probably be pretty confident of which direction the enemy will be in