Castello Sforzesco is a fortification of Milan just outside the historic center of the city.

It was built in the fifteenth century by Francesco Sforza, recently became Duke of Milan, on the remains of a previous medieval fortification dating back to the fourteenth century known as Castello di Porta Giovia. In the same area where the Castle of Porta Giovia stood, in Roman times, there was the homonymous Castrum Portae Jovis, one of the four defensive castles of Roman Milan.

Remarkably transformed and modified over the centuries, the Castello Sforzesco was, between the sixteenth and seventeenth centuries, one of the main military citadels of Europe; restored in a historicist style by Luca Beltrami between 1890 and 1905, it is now home to cultural institutions and important museums. It is one of the largest castles in Europe and one of the main symbols of Milan and its history.

Source: Wikipedia