A first church was built here in the 4th century AD, dedicated to St. Stephen. It was destroyed by a fire in 1173. The dedication to Saint Mercurialis, the local martyr and patron, had appeared since the 9th century. In 1176 the edifice was assigned to the Vallumbrosan Order. The current edifice was finished in 1180 in Lombard-Romanesque style, along with the famous bell tower. The latter, with its height of 75 m, is one of the tallest in Italy. In the 13th century it was considered one of the marvels of the Kingdom of Italy. Other points of interest include: