In Florence, as in Milan, he was commissioned to do a number of paintings, but other interests and tasks kept him from finishing them. The most notable work to survive from this period was the 'Mona Lisa', which is now in the Louvre in Paris. His largest commission, a huge mural entitled 'Battle of Anghiari' for the Palazzo Vecchio, Florence's city hall, remained unfinished.

   Other technical matters took him away from his art. For ten months during 1502, Leonardo served as military adviser and engineer under Cesare Borgia in the latter's campaign to subdue the Papal States. He traveled through Borgia's territories, surveyed them, and made sketches of city plans and topographical maps that laid the groundwork for the field of modern cartography.

   Back in Florence in 1503, Leonardo worked on the complicated engineering project of diverting the Arno River around Pisa in order to deprive that city of its access to the sea. The plan did not work, but it became the basis of a later project (never realized) to build a canal from Florence to the sea. He also busied himself with dissections of corpses at the hospital of Santa Maria Nuova, made observations on the flight of birds, and continued studies of the properties of water and its currents.