The physics behind a trebuchet is remarkably complicated too. To aim it you can:

Change the hook.
Change the length on the rope.
Change the weight of the projectile or counter-weight.
Change how much you lower the arm.
And probably more... So you can also select if you want the projectile to drop from the sky or go more horizontal.

It's also pretty amazing to see this monstrously huge thing move around on its wheels while it's firing. Without that moving, the risk of breaking down is much stronger.

I'm a little puzzled over the fact that the biggest problem seems to be to get the axle to hold though. I would think that the arm itself would be the biggest problem, as the arm needs to be as light as possible and is much longer.