The castle, which is one of the greatest monuments of the Renaissance period, was started in the 14^th century, when Galeazzo II Visconti ordered building to begin on a stronghold. It was then extended by his successors, Gian Galeazzo, Giovanni Maria and finally by Filippo Maria, who had it altered and improved with the help of the architect Filippo Brunelleschi, as he wished it to be used as the permanent residence of the Visconti dynasty. After the death of Duke Filippo Maria (1447) the stronghold was sacked by the Ambrosian Republic which had taken over the government of the city.

In 1450, the soldier of fortune Francesco Sforza, after the fall of the republic, took possession of the stronghold. He began the reconstruction with the intention of creating a fortification for his own defence but it was gradually transformed into an architecturally impressive noble residence. To start with, the work was entrusted to Giovanni da Milano with the assistance of Filippo Scorzioli and in 1451 the direction of the works passed to Jacopo da Cortona. In 1452 the Prince engaged the Florentine architect Filarete to construct and decorate the middle tower of the castle which however was begun two years later, when work on the building under the direction of Bartolomeo Gadio da Cremona resumed. After the death of Francesco Sforza (1466) his son Galeazzo Maria succeeded him and had the work continued under the architect Benedetto Ferrigni, also from Florence, to whom we owe the loggia, the great staircase of honour, the portico of the Elephant, the chapel and the rear end of the Rocchetta. The decoration was entrusted to painters of the Dukedom. Under the regency of Bona di Savoia, the tower was built to which she gave her name (1476). With the rise to power of Ludovico il Moro the fourth son of Francesco Sforza, the castle became one of the most splendid residences, decorated by Bramante, the great Leonardo da Vinci and numerous other artists summoned to work there.

After Ludovico il Moro's fall (1499), the magnificent palace was occupied by the French forces commanded by marshal Gian Giacomo Trivulzio and the beginning of the destruction of the splendid castle commenced. In 1521, a gun powder explosion caused the destruction of the central tower built by Filarete. During the Spanish domination (16^th-17^th century) the castle underwent further transformation and addition of buildings, becoming a military fortress. Charles V had a new rampart built which connected it to the new walls of the city. At the end of the 16^th century the stronghold was surrounded by six bulwarks. At the beginning of the 17^th century the moat was put in order and the covered road along the external border, and six detached ravelins were built. In 1800, Napoleon demolished the Spanish additions and only the original Sforza Castle was left standing. With the join of the Lombardy to the Regno di Sardegna, the old castle became a barracks and in 1880 was sentenced to complete destruction. During the following years, however, a large number of citizens and the interest of the Lombard Historical Society foiled all attempts in this direction, so much so that in 1893 the architect Luca Beltrami, who had already put forward a project, began a radical reconstruction. In the three nuclei of the historical building - the Parade Ground, the Rocchetta and the Ducal Court - he sited the Civic Institute for Art and History. Although it was damaged once more during the last war, the Sforza Castle was restored and became a museum.

In the middle of the façade with its front towards the centre of city rises the so-called Filarete Tower (called also the Clock Tower) which is 70 metres high. It was reconstructed at the beginning of the 20^th century by the architect Luca Beltrami, and was given back the appearance it had before the destruction of 1561. It's quadrangular in shape with two upper storeys, each one narrower than the lower, culminating in a small cupola. Above the great door is a bas-relief by Luigi Secchi representing King Umberto I on horseback (1916). Higher-up, under the first battlements is St. Ambrose amidst the coats of the arms of the six Sforza dukes. Six magnificently ornamented and richly decorated marble and brick mullioned windows are set into the powerfully structured front walls of the castle, which stretch out left and right from the central Clock Tower, leading to the two massive cylindrical corner towers dressed in rough-hewn blocks of stone. They are 31 metres high, crowned with battlements and decorated with the great marble coat of arms with the Visconti-Sforza grass snake. The sides and the rear have the same characteristics as the façade and at the level of the Rocchetta and the Ducal Court are two series of large gothic windows decorated with brickwork frames. The corner towers at the back are called Torre Falconiera to the right and Torre Castellana (or of the Treasure) to the left; they are square with large windows. At the centre of the side facing the Sempione Park there is the great Porta del Barco. On the left side, next to Porta di Santo Spirito (Door of the Holy Spirit), picturesque restored ruins of a ravelin. On the right is the Porta dei Carmini with a drawbridge and the bridge of Ludovico il Moro.

The doorway, under the Tower of Filarete, leads into the grand and picturesque Parade Ground, now a garden, was once used to exercise the Sforza troops. The internal front of the tower is distinguished by a balcony with a three-mullioned window, whilst along the left side of the wall runs a construction which act a support. Three buildings, with a dry moat in front of them, close the bottom of the courtyard. To the left stands the Rocchetta, a fortified building in which the Sforza took refuge in dangerous moments. Almost at the centre stands the Tower of Bona di Savoia 36 metres high and commissioned by the widow of Galeazzo Maria Sforza in 1477. To the right, the palace of the Ducal Court, the residence of the Sforza in time of peace and tranquillity. The solitary statue in front of the moat is St. John Nepomucenus erected in 1729. At the sides of the great square two doors with a drawbridge that crosses the external moat. The right one is Porta dei Carmini decorated with architectural fragments of different epochs. The left one is the Porta di Santo Spirito (Gate of the Holy Spirit).

The access is through the door surmounted by a great Sforza coat of arms, which rises upon the site of the old Jovian Gate and leads into the vestibule where sculptures and fragments of various Milanese buildings are kept. On the wall one can still see the fresco of the Crucifix between Saints by an unknown Lombard (1470-1480) showing the sponsor, Ambrosino da Longhirana, who was at that time the keeper of the castle for Galeazzo Maria Sforza and Bona di Savoia. From the vestibule one enters the stupendous courtyard of the Ducal Court, flanked on three sides by a two storey construction with two rows of pointed arch windows. The ground floor of the end wall is graced by Renaissance Elephant Door, which owes its name to the frescoed elephant on the wall by Benedetto Ferrini (1473). He also designed the two-storey graceful loggia at the beginning of the left wing, called the Loggia of Galeazzo Maria, that stands above the vestibule of the great staircase.

The Rocchetta is a fortress within a fortress surrounded on three sides by porticoes. The right one was constructed by the Florentine Benedetto Ferrini (1466- 1476) by order of Galeazzo Maria, the one opposite is by Filarete and the left one was begun by Bernardino da Corte in 1495 and finished by Bramante under the orders of Ludovico il Moro. From the courtyard, through an archway, one enters the Treasure Room, so called, because the ducal treasure was kept there. On the walls one can see the frescoes of the Lombard school and a damaged fresco by Bramante, showing Argus whit a hundred eyes guarding the door leading to a small room in which the most precious jewels of the Duke were kept.