One of Leonardo's largest vehicular designs was for an armored tank. The idea behind the armored car was simple: protect passengers while causing as much damage as possible. While there were, of course, no motorized vehicles in the Renaissance, the concept of an armored vehicle was not entirely new. Roman chariots, for example, were designed to help move people around and keep them safe during battle.
In his tank design, Leonardo didn't specify the powering mechanism, and his notes indicate that his tank could have been either hand-cranked (as with his automobile design) or drawn by horses. If hand powered, the cranks would have been connected to gears, which, in turn, connected to the main driving wheels.
The entire vehicle would be under a hard shell; physically, Leonardo's drawing looks like a combination of a turtle and an alien spaceship. The structure would have been clad in metal panels and, as in today's tanks, it would have had holes for guns to poke through. There appears to have been no windshield, but the two sections of the tank came apart and visibility may have been provided through the crack.
Of the two possible power sources, the hand-cranked version probably would have worked better than the horse-drawn one; in addition to being vulnerable, horses might not have remained calm enough during battle. The first “modern” tank designs did not come to fruition until the World Wars of the early twentieth century, and they definitely weren't horse-drawn!
As with the problems facing modern tanks, Leonardo's design would have suffered from a few probably fatal issues. For starters, armored tanks are large and cumbersome. They are also slow, and things which are both big and slow tend to make easy targets. Tanks are limited in the types of terrain they can traverse; Leonardo's tank would probably have had a tough time with the varied conditions of the Italian countryside.
Unless Leonardo's tank was amphibious, the canals would have posed a major hurdle. In this design, as in many others, Leonardo was far ahead of his time. The armored tank that is used in modern battles is, of course, a motorized vehicle that runs on high tread tracks. But it didn't come along for nearly another 400 years! The first armored tank to be a direct precursor to the modern tank was created in England around 1916. The idea was developed by Ernest Swinton, and designed by Sir William Tritton.