Buildings at the site of Somma Lombardo Castle date from the 9th century and the fortress from the 13th century. In 1448, the brothers Francesco and Guido Visconti, took refuge in this town and castle from the forces of the Ambrosian Republic. They divided the landholdings and portions of the castle. The fortress has been amalgamated from adjacent residences, all once surrounded by a single moat. From the brother Francesco, who owned the newer portion of the castle, descend the Visconti di San Vito; while from Guido, who owned the lower and older portions of the castle, descend the Visconti di Modrone.
The San Vito castle contains frescoes attributed to the school of Camillo Procaccini and an altarpiece by Cerano. The castle also has an extensive collection of barber plates.
Parts of the castle are used for private events and open for guided tours.
References:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.