The “Castellazzo” or castle complex of Romagnano Sesia is an ancient medieval fortress at the ford of the River Sesia. The complex was once an important garrison for controlling and defending the area, and has the typical features of castles built before the year 1000.

The Castellazzo comprises two encircling walls on a quadrilateral plan; the first wall supports the internal buildings of the fortress, while the second, narrower wall features interesting splayed slit windows, crenellations and traces of walkways. The walls of the massive structure are typically built with irregularly arranged river pebbles.

The Castellazzo stands near the 11^th century Romanesque Church of San Martino.