Leonardo’s design for the catapult is both simple and ingenious. It involves a pawl and rachet system which incrementally tightens the firing system. As the system is tightened, the forces put into the system by the operator (1 soldier) are transferred to both the ropes and tension arms of the catapult. Upon releasing the firing pin (the pawl), the stored energy is instantly transferred from the ropes and tensioning arms to the swing arm which would contain a lead ball/cannonball.
This machine looks like it cound have been operated by a handful of soldiers : four or five soldiers for moving/repositioning/aiming the catapult and 2 men for operating the machine – one to tighten the mechanism and one to release the firing pin.
This machine would undoubtedly have wrought havoc upon any castle/fort it attacked. It could be reloaded and retightened very quickly. Of course, as with all medieval catapults, the soldiers may have decided to fire flaming ballistics or even disease-ridden dead bodies into the castle they were attacking – this usually spread either fire or disease very quickly within the walls of the target castle/fort. Within a castle that is under sustained attack, water supplies quickly begin to dwindle and the population begin to grow weak due to lack of food, thereby decreasing their odds of putting out any flames and increasing their odd of succumbing to illness from disease-ridden bodies flung over the walls.