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A ballista is a siege weapon which resembles an enormous crossbow. King Foltest had given the Baroness La Valette some ballistae as a birthday present two years prior and later found himself on the receiving end of said gifts.
These weapons with arms strengthened with bronze plates and strung with horsehair are capable of propelling a heavy bolts up to a mile away, but because of their sheer strength, the recoil of each shot causes the weapon to shift from its original position, so the same target cannot be hit on two consecutive fires. An experienced crew can cock and release two bolts each minute. It is a deadly and very costly weapon.
In the Prologue, Geralt helps Foltest's soldiers aim one of their own ballistae at Count Etcheverry. He then later uses one himself to take out some of the La Valette troops on one of the towers.
In Chapter I, Vernon Roche asks him to sabotage a different ballista in the compound outside Loredo's residence.