Battlefield role
Long-range bolt thrower that threatens fortifications, massed units, and large flying targets.
The ballista is Castle Age problem-solving: walls too thick, formations too dense, or a dragon too comfortable in the sky — a bolt the size of a fence post is a persuasive counterargument to all three.
Everything about siege play is protection math. A screened ballista dictates the fight from range; an unscreened one is a gift to the first cavalry that finds it. Where your ballistae stand is where your army has decided to fight.