The Province of Milan (Italian: Provincia di Milano) is a province in the Lombardy region, Italy. Its capital is the city of Milan. The provincial territory is highly urbanized, resulting in the third highest population density among the Italian provinces with more than 2,000 inhabitants/km2, just behind the provinces of Naples and the bordering Monza e Brianza, created in 2004 splitting the eastern part from the province of Milan itself.
The Naviglio Martesana canal starts from the Adda in Trezzo's territory.
Trezzo sull'Adda borders the following municipalities: Cornate d'Adda, Bottanuco, Capriate San Gervasio, Busnago, Grezzago, Vaprio d'Adda.
Trezzo received the honorary title of city with a presidential decree on 8 July, 2008.
Trezzo's main attraction is the massive castle which belonged to the powerful Visconti family in the 14th century. Protected by the Adda on two sides, it had a 42 m-high square tower on the third one. Its fortified bridge (see Trezzo sull'Adda Bridge) was 72 m long, the longest bridge span for several centuries, built on three different levels, passing 25 meters over the waters.
Due to its strategic position, the castle was contested first by Emperor Frederick Barbarossa and Milan, in the 12th century, and later by the Visconti and the Torriani families. The castle was destroyed or burnt several times, but was rebuilt on each occasion. The last reconstruction dates from 1370. It was commissioned by Bernabò Visconti, who was later imprisoned there until his death.