In an article on the date of Machiavelli's Mandragola published recently in this journal, Sergio Bertelli makes reference to an unpublished document concerning Leonardo da Vinci as a military architect. This is a letter that the field commissary Pier Francesco Tosinghi wrote to the Florentine Republic on June 21,1503, reporting Leonardo's visit to the fortress of ‘La Verruca’ on the summit of the Verrucano mountain, about three miles from Pisa. ‘Leonardo da Vinci,’ writes Tosinghi, 'himself and company, came here and we showed him everything, and we think that he likes La Verruca very much, being well fitted to his taste. Afterwards, he said that he was thinking of having it made inexpugnable. But this is something to be set apart for the time being, for the main need is at Librafatta, which is not a small undertaking, nor one to be underestimated.