Video: In his spare time -- which was presumably limited -- Leonardo da Vinci was fond of drawing plans for contraptions and weaponry, most of which were never actually built. But with a few common items, it's easy to bring his double-armed catapult to life. Even at such a small scale it's easy to see that da Vinci was on to something. His design isn't nearly as heavy, cumbersome or difficult to fire as, say, a trebuchet. But elegant as they may be, there's no historical record of these siege weapons ever being made during da Vinci's lifetime. Probably because the discovery of gunpowder made this a pretty unsexy prospect.
Although these particular plans from Mist8k are scaled down for desktop use, the principle is sound at larger sizes. That said, we're not responsible if you send a bowling ball through your neighbour's roof.