The typically Renaissance subject of the ideal city fascinated Leonardo who began to concern himself with it in Milan in the late 1480s. Unlike contemporary treatise writers, his study of the organisation of space was less geometrical then functional, and directed at solving the various problems of daily life, such as traffic, supply, and hygiene and sanitary requirements. The last of these played a part of primary importance in Leonardo's plans and it is no coincidence that his study of the ideal city dates from the aftermath of a serious plague epidemic, which struck Milan in 1485. The structure of late Medieval cities greatly encouraged the spread of infection with their narrow twisting streets, high population density (especially in the poor quarters), open sewers, little personal hygiene and a great number of rats and parasites. To find a solution to this highly explosive situation as regards sanitation, Leonardo proposed a much more open city plan, characterised by wide, straight streets, with a comprehensive system of waterways.
According to him, cities should be built near rivers that are sufficiently fast-flowing not to create areas of stagnant water that can pollute the air. By means of sluices and basins, the river water would be led into the city by a network of canals, which would make it possible, above all, to provide urban cleaning and the removal of sewage, for which he designed a proper, underground sewage system. Besides this essential aspect of hygiene and sanitation, the canals would also perform other important functions such as ensuring communications and facilitating supplies. Goods traffic would be carried out, at least in part, on water, and organised in such a way as to allow the unloading of goods directly within individual buildings, with some of those having semi-basement warehouses with direct access from the external canal through a little dock. The canal network would also be integrated into a carefully organised street system which would include streets for vehicular traffic and the common people and, at a higher level, streets exclusively for the movement of the upper classes.
The city would therefore be articulated on two levels with strict separation between production operations and upper class concerns, reflecting the structure of aristocratic palazzos. The family of the owner of the building lived on the first floor, which would be entered from the upper street, while the lower floor was for service areas (kitchens, storeroom and wood store) opening on to a wide courtyard that would communicate with the lower street. Like all such plans of that time, and despite its strength as regards functionality, Leonardo's project was not capable of being realised and was abandoned. The originality of the project, however, and its potential for being synthesised in the fusion of architecture, mechanics and hydraulics, as well as the idea that the beauty of a city must be synonymous with functionality means that, with Leonardo, the idea of modern city-planning was consolidated.