The relations between Leonardo and the Governor of Milan, Charles II Chaumont d’Amboise, during the French occupation of Milan in 1506–1507 are well known through the letters addressed by the latter to the Florentine Republic in 1506–1507 and from the letters written by Leonardo to Charles d’Amboise. Gerolamo Calvi was the first to suggest that Leonardo may have executed some architectural works for the ‘gran maestro’ (this was indeed the title of Charles d’Amboise in 1506). Afterwards Carlo Pedretti reconstructed Leonardo’s projects for a villa, on Charles d’Amboise’s commission, but this hypothesis was recently questioned by some scholars, such as Pascal Brioist and Laure Fagnart, who suggested that Leonardo was only charged with the organization of a scenic apparatus or temporary works, without involvement in architectural and practical engineering works. However, recent studies by Marino Viganò indisputably link Leonardo to the military survey on the border between Lombardy and Switzerland in 1507, when Leonardo was guest of Charles d’Amboise and when he probably lived (as he had before 1507) in the latter’s house in Milan. Precisely, in the first days of June 1507 Charles d’Amboise offered a performance in the garden of his palace in honour of the King of France, Louis XII, consisting in an assault on a bastion which is described by Jean d’Auton in his Chroniques de Louis XII: ‘ung bastyon […] fossoyer tout autour, et fermer de gros boys debout mys en terre et au devant tout a l’environ fortiffyé de planchon a groux et chevilles bien actachées’. This bastion had also two towers at its corners and its (clearly evident) triangular shape with two towers corresponds not only to some images of bastions drawn by Leonardo in the Paris Ms. B, c. 1487–1490, but also in his later studies from these years, i.e. Codex Atlanticus, fol. 117 recto (showing a fortress, probably in relation to an addition of a ‘rivellino’ promoted by d’Amboise in Locarno in 1507) and in other studies of earthen bastions, where willow trees had been planted, found in Ms. Arundel in the British Library and in Paris Ms. K. Because of the use of earth (given the needs to construct the bastions very quickly) in turn, these Leonardo’s studies anticipate the drawings of earthen bastions Michelangelo executed in 1528–1529 for the defence of Florence. It is logical to assume that d’Amboise, who hosted Leonardo in his home and had employed him as an architect at least from the previous year, asked him to design this bastion, perhaps similar to the one he had made for him in Locarno earlier that year. This occurrence, in turn, call also for a reconsideration of the role of Leonardo as the personal architect of d’Amboise; what is more, it is also possible that he designed a new villa for him. It is not by chance that in the same year, on 26 July 1507, Leonardo was appointed ‘nostre paintre et ingenieur ordinaire’ by the King of France.