Leonardo's cartography pertinent to the deviation of the Arno reaches, in its southernmost part, as far as today's San Vincenzo, in the Province of Livorno. But the Tuscan coastal locality that he reproduces most faithfully is that of the promontory of Piombino. On map RLW 12277 Leonardo delineates the profile of the peninsula between the Gulf of Baratti, the "Stagno" (marshes) of Piombino and the Valle del Cornia. He also notes his observations on the territory and projects for the fortification of Piombino in Ms. L, in Madrid Ms. II and in the Codex Atlanticus.

Taking into consideration the advice of Machiavelli, Florence had ended by recognising, for a brief time, Cesare Borgia as "Lord of Piombino" after his troops had conquered the city, taking advantage of the momentary absence of its sovereign, Iacopo IV degli Appiani. It is probable that certain rapid annotations, such as those of Ms. L, refer to a first period (1502) spent by Leonardo in the environs of this strategic port (the ancient Falesia of the Romans) and on the coast between Populonia and Follonica, to study methods for draining the swamps and new solutions of military engineering.

It is however certain that, in 1504, Leonardo worked in Piombino for Jacopo IV Appiani (with whom Florence was now seeking alliance, or at least neutrality). Although his initial responsibilities were limited to the indications stipulated by Machiavelli, Leonardo went far beyond them, drawing up complex studies resulting in a treatise on architecture that appears as trait d’union between that of Francesco di Giorgio Martini and the theoretical works of the early fifteenth-century engineers.

It is interesting to find that some notes on navigating by sailboat show that Leonardo studied together with Antonio da Sangallo the Elder at Piombino. On folio 122r of Madrid Ms. II, the words «luff to the front, that is, at the bow» and «luff to the stern» appear in the handwriting of Sangallo. Carlo Pedretti has traced this relationship also through architectural drawings found in Ms. L and in the Codex Atlanticus (such as folio 115v, with measurements for new bastions to defend to citadel).

A description of the strategic motivations and the costs required to reinforce the defensive system, the reason for which Leonardo had been called to Piombino, is repeated several times in the same Madrid Ms. II, for example on folios 37v and 38r-v: «Under this wall we will dig an underground hollow, that is, a covered road, for the citadel to come to the aid of the barbican at the gate, in which barbican the covered road will enter with a drawbridge, so that when the people have surrendered to the enemy, the earth can be broken over said covered road, making it impossible to conquer the barbican.»

Leonardo calculated the «sum total of the costs of said construction», which included the moat, the walls of the moats, the parapet and the tower («The aforesaid tower has a diameter of 25 braccia and a circumference of 78 and 41/7 braccia and its height is 20 braccia, all massive and of uniform thickness down to its base [...]»). The sum total amounted to 2099 ducats, to which had to be added another «3000 lire» for «levelling the hill» and for the «hill that remains along the marina», «that is 428 and 4/7 ducats, at 7 lire per ducat, which, added to the aforesaid 2099 1/16, make the sum of 2528 ducats.»

Of very different nature are some notes on the subject of painting made during his stay in Piombino, in which calculations alternate with curious observations such as those concerning the perceptive faculties of animals: «Darkness is the absence of light. Shadow is the diminution of light. [...] As regards seeing and hearing, the cat has the keenest senses of all animals. And its sense of smell is almost equal to the aforesaid senses. And where visual power is insufficient, it recurs to the sense of hearing, always keeping its ears erect, like a funnel, to receive the impression of the noises arriving on the air, and to send them by broad pathways to the Common Sense.» Knowing how Leonardo worked, the presence of such digressions on sheets filled with calculations and lists of expenses is not surprising.

The same can be said of some notes of poetic, as well as scientific nature, such as the one appearing on folio 125r: «1504 at Piombino, the day of Ognissanti. On painting. I have already seen the greenish shadows cast by the ropes, mast and spars on a white wall, as the sunlight struck it. And it happened that the wall surface which was not lit by the sun took on the colour of the sea that was before it.»

In Ms. L, Leonardo studied la Rocchetta, the «way to dry the marshes of Piombino», and the profiles of the promontories of Populonia and Piombino. In Madrid Ms. II, on folio 89r, he reprises the theories of Francesco di Giorgio: «If a square tower will have, at the corners, pyramids, with various recesses, such as stairways, doors and double bridges, back entrances, barbicans and moats, it will be in itself highly resistant. In this port let us place, at a distance of 250 feet from its entrance, a wall 80 feet high, and 300 feet long, with a sloping escarpment. This port has no adverse winds, because the wall a-b serves as fortress and bulwark against the waves, and as a lighthouse to be secure against any wind. This other has 2 floodgates on the part of the land walls, which when they open, will let the sea cleanse and rinse the port of all ordure.» This passage continues on folio 88v, where the model derived from Francesco di Giorgio finds its definition ("Port of Piombino") along with technical indications for building the structures in water: «When it must be sunk in the water where there is no solid stone, make 2 circuits of piles [...]». On folio 9r we find references to the tower and the moat (with new bastions and tunnels connecting to the "barbican at the gate"). On folios 24v and 25r – amid projects for bastions, studies on levelling a hill and on building a firing range, the section and plane view of a new quadrangular fortress – we read: «On the last day of November for Ognissanti 1504, I demonstrated this in Piombino to its Lord". He also describes how in the "Castle keep of Piombino on the day of November 20, 1504», his first project consisted of a "covered road" between the existing citadel and the new tower (designed by him) and a "straightened ditch" (a high gallery) between the new tower and the main gate of the city, as well as a trench about 380 metres long between the citadel («the fortress on Mount Sancta Maria») and the Rocchetta. On the same folio 35r are found calculations for the fortifications of Piombino: «Moat, hills, walls and enclosure of the fortress on Mount Sancta Maria, amount to 5942 ducats. Moat and wall of this moat, and defence of the land with the tower and its buttresses, 25 braccia thick and 20 high, all included, apart from the defence, amount to 2393 ducats.»

Still at Piombino, Leonardo studied waves, sailboats, winds and navigation, along with trajectories and a firing range for artillery, as well as the advisability of changing the height of the hills around the fortifications. And from here he observed the Island of Elba.

In the Codex Atlanticus are projects for fortifications, extraordinary from the aesthetic and architectural viewpoint, which can be dated to the early years of the 16th century, between the time when Leonardo was working for Cesare Borgia and his stay in Piombino. Here, in the Citadel, Leonardo's interventions are still recognisable today: for example, in the northern wall with the moat, in the buttresses (to compensate for settling of the terrain), in the wall at the western end of the land front. The Rivellino (barbican) at the Land Gate is noteworthy.

The exactness of the calculations and the surveys (performed with odometer and compass), of the maps and the projects on Leonardo's folios from autumn 1504, is remarkable.