The construction, perhaps on Visconti remains, of the ravelins, projections placed to protect the gates, began with the renovation of the Castle by Francesco Sforza. Of the Sforza ravelins, two bases remain on the east side of the Castle, one to defend the Porta del Carmine, the other where the medieval walls of the city were grafted and to the west, the Rivellino di Porta Vercellina. The latter, built in correspondence with the Porta di Santo Spirito, is the only one to have maintained the visible Sforza quadrangular plan, even if the current appearance is the result of the restoration by the architect Luca Beltrami. In fact, between 1911 and 1914, Beltrami restored part of the battlements, demolished at the time of the Spanish, the walls and the vaults of the passages and internal rooms. To defend the main gate of the Castle, Leonardo designed, commissioned by Ludovico il Moro, a more modern, pentagonal ravelin, which was built during the French domination of Louis XII, between 1499 and the beginning of 1500. Few traces of this bastion remain hidden in the moat.