The site of the Castelgrande has been fortified since at least the late 1st century BC and until the 13th century it was the only fortification in Bellinzona. During its history the castle has been known as the stronghold (before the 13th century), the Old Castle in the 14–15th centuries, Un Castle after 1506 and Saint Michael's Castle from 1818.
The Castelgrande hill includes a nearly vertical side on the north and a steep southern side, but is nearly flat and 150-200m in diameter. The natural shape of the hill has encouraged every man-made fortification to follow the same contours. While the Roman fort is not visible the Roman foundations were used by the High Middle Ages castle which followed. Of the High Middle Ages castle the only visible parts are a few pieces of wall that are still standing. Much of the visible castle dates from 1250–1500 with extensive renovations and some expansion in the last two centuries. Most of the area inside the castle walls is now flat, open space.
Records from the 11th to 15th centuries as well as archeological evidence indicate that the castle grounds were once full of buildings. However most of these were pulled down by the Dukes of Milan to free up interior space. The open space was divided into 3 large baileys which served to provide temporary housing for troops that could be stationed in Bellinzona. Under the Dukes of Milan the outer fortifications were extended and strengthened. The walls were raised, extended and towers were added. The western walls were totally rebuilt and connected to the city walls.
The castle can be reached by taking an elevator from the foot of the rock to the castle grounds or by climbing steep, narrow streets from the city center through the city wall onto the castle grounds.
