Leonardo indicates by "Verruchola" (or "Verrucha") the strategic fortress and the mountain of Verruca in the Commune of Calci. He draws it, around 1503, on folios 4r, 7v and 8r of Madrid Ms. II and on Windsor paper RL 12683; and also mentions it on folios 1r, 1v, 3r, 22v, 53r of Madrid Ms. II, and on folio 305r of the Codex Atlanticus. The place is perhaps evoked in idealised form also in a drawing of an "Apocalyptic flood" dating from around 1516 (Windsor, RL 12385).
The fortress, clinging to a rocky outcrop, was one of the cornerstones of the Pisan Republic's defense system on the Lucca side together with the castles of Caprona and Vicopisano. Scipione Ammirato calls it "a reef that looks like a herd of bulls". In a letter dated June 14, 1503, Niccolò Machiavelli suggested to the captains of the Florentine army, which was besieging Pisa: "... Remove from before your eyes Verrucola, which has always been a constant harassment and impediment to these places of ours in the interior, and of no little aid to our enemies ... And thus that which had to be done, by attacking Verrucola, and expunging it". It was only seven days later, on June 2, that Leonardo went to inspect the fortress, already conquered, and to plan its restoration. In the documents relevant to the two following weeks, he mentions drawings and a model for works that had presumably already been completed by the following year, when Antonio da Sangallo the Elder went there (on June 7, 1504), with his experience of the fortifications of Ripafratta, "to see if anything is missing".