Trezzo's main attraction is the massive castle which belonged to the Visconti family in the 14th century. Protected by the Adda on two sides, it had a 42-metre (138 ft) high square tower on the third one. Its fortified bridge (see Trezzo sull'Adda Bridge) was 72 metres (236 ft) long, the longest bridge span for several centuries, built on three different levels, passing 25 metres (82 ft) over the waters.