Sforza Castle

The original construction was ordered by Galeazzo II Visconti, a local nobleman, in 1358 – c. 1370; this castle was known as the Castello di Porta Giova (or Porta Zubia), from the name of a gate in walls located nearby. It was built in the same area of the ancient Roman fortification of Castrum Portae Jovis, which served as castra pretoria when the city was the capital of the Roman Empire. It was enlarged by Galeazzo's successors, Gian Galeazzo, Giovanni Maria and Filippo Maria Visconti, until it became a square-plan castle with 200 m-long sides, four towers at the corners and up to 7-metre-thick (23 ft) walls.

In 1450, Francesco Sforza, once he had shattered the republicans, began reconstruction of the castle to turn it into his princely residence. In 1452 he hired the sculptor and architect Filarete to design and decorate the central tower, which is still known as the Torre del Filarete. After Francesco's death, the construction was continued by his son Galeazzo Maria, under the architect Benedetto Ferrini. The decoration was executed by local painters. In 1476, during the regency of Bona of Savoy, the tower bearing her name was built.

After the French victory in the Battle of Marignano in 1515, the defeated Maximilian Sforza, his Swiss mercenaries, as well as the cardinal-bishop of Sion retreated into the castle. However, King Francis I of France followed them into Milan, before his sappers placed mines under the castle's foundations, whereupon the defenders capitulated. In 1521, in a period in which it was used as a weapons depot, the Torre del Filarete exploded.