Military Engineering Exploits - Giant Crossbow

Regarding the size of this crossbow, Leonardo writes that it "opens its arms 42 arm lengths" or approximately 24 metres (the stretch of the strings) and is mounted on a cart "2 arm lengths wide and 40 long". The size of the archer in the diagram gives an idea of the proportions.
The bowstring is pulled tight by a screw mechanism as shown in the bottom right of the diagram. The wheels tilt to offer a more stable base for firing.