The castle of Capraia was drawn by Leonardo on the southern slopes of Montalbano, in the bird's eye map RL 12685 of the Windsor collection, which represents in detail the stretch of the Arno west of Florence. Its appearance is that of a fortified hill, surrounded by a curtain wall fortified with eight wall towers. The same characteristic details can be found in the less well-known, recomposed drawing from the Codex Atlanticus, folios 765v-766r, where he depicted, with a stroke of red chalk, the profile of Montalbano seen from the bend of the Arno, with the castle of Capraia on the right and the one in which, on the left, perhaps, a part of Montelupo can be recognized. Leonardo knew very well these places and their nature: he mentions Capraia and Montelupo in several passages of his observations on the origin of the geological formations of the Lower Valdarno.
Leonardo's scientific observations in the field of geology and paleontology are mainly found in the Codex Leicester, and concern various parts of Tuscany. Several passages of the codex concern the area of Capraia, where Leonardo had observed the conglomerates of Plio-Pleistocene origin that he called "ghiare" (gravels). Leonardo demonstrated a level of interpretation of the phenomenon representing considerable acumen, with respect to the geological sciences of his time, which were still stuck on the theories about the Great Flood; he illustrates with great clarity the origin of those geological formations that he calls, on the basis of the dimensions of the individual inclusions, "gravels", "sands", and "muds". The movement of the waters of the sea, which once came to lap the slopes of Montalbano, up to La Gonfolina, was the cause for the formation of the gravels and the sands, fragments of rocks brought to the shore of the Valdarno sea by rivers, and made roundish, though with different caliber of nuclei, by the motion of backwash. This can be seen, according to Leonardo, in different parts of Italy, from the Alps around Como, to the Po River Valley, and even in the Arno Valley, on the slopes of Montalbano, around Montelupo and Capraia (Codex Leicester, ff. 6A-31v.). Leonardo continues, to explain that the Arno at that time flowed into the sea at the position of La Gonfolina, depositing on the slopes of Montalbano the conglomerates of pebbles and gravels: “come Arno, che cadea della Golfolina a presso a Monte Lupo, e quivi lasciava la ghiara, la quale ancor si vede che s’è insieme ricongielata "like the Arno, which descended from La Golfolina, to nearby Monte Lupo, and there it left the gravel, which we can also see has been reunited together."