Dionysius of Alexandria invented this wonder of ancient Greece mechanics. The catapults accuracy has not been beaten in Ancient Times. We do not know if it was used on greater occasions.
The only thing one has to do is to turn the wheel back and forth. The wheel moves two chains, which in turn moves a slider.
On the slider lies the arrow. A claw keeps hold of the bowstring. The claw lifts, when a joint is rotated. The arrow flies away.
An arrow magazine is attached on top. The arrows make their way through a rotating tray with cam groove. The tray rotates, when the finger moves back and forth inside the cam groove