I have no watched, on EVERY SINGLE siege, the sieger attempt to build offensive weapons such as catapults and ballista. The result? Dead in seconds. The defending city/castle either builds their own defenses so fast, or already has them, so that as soon as a single weapon is built, it is targeted and destroyed by the weapons defending the location. these weapons in an ACTUAL seige would be constructed in safety and then moved forward when it was time to assault. Further, a trebuchet is meant to have far superiro range than an onanger, so it shouldn't be able to be hit by the defensive weapons.