Montebello Castle (known as the Small, New or Middle Castle in the 15th century, as Schwyz Castle from 1506 and St. Martin's Castle after 1818) is located to the east of the town center. It was built before 1313 for the pro-Imperial Rusca family, who occupied the castle following the Visconti victory and occupation of the Castelgrande. By the end of the 14th century it was in the hands of the Visconti. The castle was renovated and expanded between 1462 and 1490 to its current state. In the 19th century the castle fell into disrepair and was renovated starting in 1903.
A little chapel, dedicated to Saint Michael, leans against the wall of the more recent south-facing section; built around 1600, it is one of the few buildings erected in the castles of Bellinzona under the rule of the three Swiss cantons.
Montebello Castle houses the Archaeological and Civic Museum. The museum was opened in 1974 and is located in the tower and the former residential quarters of Montebello Castle. It is divided into two sections-history and archaeology. In the history section there are several capitals from the 15th century and a rare 13th-century Baptismal font as well as drawings and sketchs from several artists. This section also houses a collection of ceremonial and military arms. The archaeology section includes many items from 1400–1500 BC as well as ceramics, glassware, funeral urns, ornamental objects and jewellery in iron and bronze from around the canton. The museum is open from March to November.