After a plague had struck Milan, Italy and killed much of the population, Leonardo da Vinci created a plan for a city that could prevent the massive spread of disease. His design for this "Ideal City" was to build it on two levels, the lower level for animals and tradesmen and the upper level would be people walking. There would be canals below to take away waste and garbage, streets would be wider, and overall there would be better living conditions. His intent was to make a city with improved sanitation and lower the chance for a diease spreading like it had, taking out a large portion of the population. Image source: Wikimedia Commons, listed as public domain in the United States because the copyright term has expired.