Having built the trebuchet a couple of years ago, last year I attempted a torsion powered ballista, but didn't get it to work. That one is now on the back-burner while I rethink it.
This year however, a torsion powered mangonel is the job. Found a good bit of 2x4 in the shed, and a couple of narrow bits of ex-shelf of the perfect length and thickness.
I will post pics eventually, but for now I have a substantial ladder frame base about 2ft by 1ft, will be adding uprights and spreader at the top, plus supports for the frame. I've got a 16mm spade bit for the cords to pass through with washers to reinforce. Haven't quite decided on the arm or bucket yet, but have plenty of options. Will also add a simple trigger. Daughter off school now, so will try and get her involved as a physics exercise.