In 1479 Florentine garrisons came under siege and Leonardo worked, both on systems for scaling walls, and on defenses against people trying to launch an assault on the city.
There were a number of designs for scaling ladders with some being simple rope ladder designs whilst others were rigid. Some had grappling hooks attached for hooking them onto the walls, others had spikes which attached them firmly into the ground. One interesting design shows a man crossing a moat using a system very similar to that used to climb into the rigging of old sailing ships. Leonardo also considered the problem of how the ladders would be taken to the tops of walls and devised the idea of a man climbing up using pitons and dragging a rope ladder behind him.
Obviously defence against this sort of attack was also required. In one design Leonardo had a large cog which, when set in motion, simply swept the soldiers from the walls by unbalancing them. Another system could simultaneously push a number of rigid assault ladders away from the wall until they fell backwards. The horizontal bar on the outer wall could be easily operated by several men on the inside, by turning a winch or pulling on a rope.
In his letter to Ludovico Sforza, Duke of Milan, Leonardo wrote that:
1. I have plans for bridges, very light and strong and suitable for carrying very easily, with which to pursue and at times defeat the enemy; and others solid and indestructible by fire or assault, easy and convenient to carry and place in position. And plans for burning and destroying those of the enemy.
2. When a place is besieged I know how to cut off water from the trenches, and how to construct an infinite number of bridges, battering rams, scaling ladders, and other instruments which have to do with the same enterprise.
Leonardo was not exaggerating and his documents contain designs for a number of bridges. There is an arched bridge which gains its strength from the shape. There are military flying bridges used for crossing rivers. To counter the problem of river banks that were of different heights Leonardo included adjustable jacks. Some bridges were mounted on pillings, while others were swing bridges. They were designed to be light, manoeuverable and quickly assembled.