Cesare took him into his employ as engineer and architect, leaving him in the Romagna for the present.

Leonardo may have superintended the repairs of the Castle of Forli, whilst he certainly built the canal from Cesena to the Porto Cesenatico, before rejoining the duke in Rome.
On May 25 Cesare moved by the way of the valley of Cecina to try conclusions with Giacomo d'Appiano, Tyrant of Piombino, who with some Genoese and some Florentine aid, was disposed to offer resistance to the duke.

The first strategic movement in this affair must be the capture of the Isle of Elba, whence aid might reach Piombino on its promontory thrusting out into the sea.

For this purpose the Pope sent from Civita Vecchia six galleys, three brigantines, and two galleons under the command of Lodovico Mosca, captain of the papal navy, whilst Cesare was further reinforced by some vessels sent him from Pisa together with eight pieces of cannon.

With these he made an easy capture of Elba and Pianosa.