Town planning and surveying was another field of activity for Leonardo. His drawings for a city served by canals was probably prompted by calls for improvements following Milan’s disastrous plague of 1484-85, which killed some 50,000 people (one-third of the city’s population). Milan and its region was famed for canals, but Leonardo now proposed a two-level canal and street network with barges transporting heavy goods, and a separate upper street grid for “persons of quality” untroubled by heavy wagons, cleaned by the rainwater draining into the canals below, and serviced by public latrines and by spiral staircases connecting the two levels. This was an age of ideal city designs as well as the launch of major urban developments such as Lodovico Sforza’s Piazza Grande in nearby Vigevano, and the Duke of Ferrara’s urban extension, the so-called “Addizione Ercole”, both started in 1492. Leonardo’s somewhat earlier city planning proposals for Milan are generally treated as a pragmatic response to dirt, disease and traffic congestion, rather than as blueprints for an ideal urban future; but his interest in canals, land drainage, water mills, pumping systems and hydraulics was to last his lifetime, culminating in proposals for a canal linking the Loire river system with a new town which he had planned for the King of France at Romorantin, near where Leonardo was to spend his last years. When the royal new town project was abandoned, Leonardo may well have contributed to the design of the central keep and double spiral staircase at the Chateau of Chambord.

Military machines and siege equipment are another continuous Leonardo theme, and a series of drawings (dated to the years 1497-1502) explore designs for fortifications with curved surfaces designed to deflect the impact of cannon balls fired by the gunpowder artillery that was then transforming warfare. The French invasion of Italy in 1494-95 set in motion a series of campaigns which removed Lodovico il Moro into French captivity and forced Leonardo to look for new employment in Florence. When he returned to Milan in 1508, it was occupied by the French. His most advanced fortification studies therefore coincided with real opportunities for military building in war-torn Italy.

Documented commissions from Cesare Borgia in the Romagna, the Appiani rulers of Piombino, and the Venetians at Gradisca all date from the turn of the century; but here too there is no firm evidence of completed works, and no opportunity to compare either new or modified works with the sketchbook designs. Leonardo’s fortifications – often circular in plan, and sometimes star-shaped in ways which can be seen to anticipate later developments – have been dismissed by some modern writers as fanciful and impractical. However they incorporate features encountered in the sketchbooks of men such as Francesco di Giorgio and the older Sangallo brothers who pioneered the new military architecture of Renaissance Italy. One of his schemes for low level pill-boxes around the base of a circular keep even re-appears in the drawings of Antonio da Sangallo the Younger who, in the 1530s, was planning a new fortress for Florence. Informed contemporaries clearly took Leonardo’s fortifications seriously. Perhaps we should do the same.

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