Leonardo da Vinci's weapon was based on the principle of the ancient ballista, but on a gigantic scale, with a laminated wooden bow mounted on a large multi-wheeled frame for mobility.
Systems of cranks, gears and ropes were used to draw the bow and release the shot, probably to launch stone blocks or incendiary projectiles rather than simple arrows.
The project was not built during Leonardo's lifetime, but modern 1:1 scale reconstructions have shown that it was technically feasible.