In 1515 Pope Leo X (Giovanni di Lorenzo di Piero de’ Medici, 1475-1521), while travelling to Bologna to meet the new King of France, Francis I (future patron of Leonardo), stopped in Florence, at Santa Maria Novella and then at the Medici Palace in Via Larga. On November 30th the city held extraordinary festivities in his honour. Old and new acquaintances of Leonard's such as Baccio d’Agnolo, Piero di Cosimo, Pontormo, Andrea del Sarto, Francesco Granacci, Jacopo Sansovino and Antonio da Sangallo il Vecchio, also collaborated on the scenography and organisation of the event.
At this time there arose the idea of rearranging the section of Florence between Piazza San Marco and Piazza San Lorenzo and completing the facade of the San Lorenzo church.
Leonardo worked on a project consisting of several parts, including the following initiatives: constructing a Medici Palace, opposite that of Cosimo the Elder (today's Palazzo Medici Riccardi); demolishing the Church of San Giovannino (built in 1351 and still existing today, with the facade rebuilt around 1843, in Via Martelli on the corner with Via Gori) and rebuilding it in front of the new Medici Palace; providing a facade for San Lorenzo and enlarging the square in front of it; a new arrangement of the San Marco area, including the Medici Stables, which Leonardo called, in the Codex Atlanticus, "Stables of the Magnificent", because they had been built for Lorenzo di Piero de’ Medici (nephew of the pope, elected Governor of Florence in that same 1515). Between 1515 and 1516, a building between San Marco and Santissima Annunziata that matched Leonardo's sketch was in effect constructed. And in the mid-sixteenth century the Lion Menagerie was moved to the former Via del Maglio (since 1877 Via Lamarmora, in the section recently renamed for Giorgio La Pira). In 1592 Ferdinand I de’ Medici had the present-day stables built. Still today the architectural complex, which includes the Rectorate of the University, the Istituto Geografico Militare and the Santissima Annunziata Convent, constitutes an exceptionally interesting area for seeking out and visiting the places linked to Leonardo.
Other architects too worked on these projects, such as Antonio da Sangallo, Baccio d’Agnolo, and above all Michelangelo, who was commissioned to design the facade of San Lorenzo, never realised (the wooden model is now in the Casa Buonarroti museum in Via Ghibellina).
In the place where Leonardo, around 1515, had planned to build the new Medici Palace, the present-day Palazzo Panciatichi (seat of the Regional Council of Tuscany) and Palazzo Capponi-Covoni were built and subsequently remodelled.
The latter is especially interesting for the intricate relationships between its various owners, Leonardo himself and historic Florentine families such as the Tani and then the Portinari, directors of the Medici Bank in Bruges, through whom Leonardo asked information on ice-skating in Flanders; and still others, such as Andrea di Paolo Carnesecchi, Consul of the Florentine Republic at Constantinople at the time when Leonardo was designing the bridge for the sultan of that city, and indirectly the Capponi family, descendants of that Neri di Gino Capponi who, «being General Commissioner of the Florentines defeated the armies of the Duke of Milan at the Battle of Anghiari» (portrayed by Leonardo in the "sumptuous cartoon" drawn in Santa Maria Novella and recorded in the 18th century in Palazzo Medici Riccardi). These traces are even more evocative considering that the seat of the Regional Council and the "Chanto di Via Largha" are only a few metres away from the house of Piero di Braccio Martelli (today the seat of the Liceo named for Galileo, adjacent to the Ximenian Observatory, an institute specialized in meteorology and geophysics), in which Leonardo spent many years of his second Florentine period.