Although popularly called a cathedral, it does not have that status, as it is under the Diocese of Milan. The Lombard queen, Theodelinda, built a royal chapel, an oraculum, in 595. The present church was built on the remains of the chapel in 1300. In the late 14th century, the side chapels were added and, as designed by Matteo da Campione, the Pisan-Gothic style west front in white and green marble was begun. The bell tower was erected in 1606 to designs by Pellegrino Tibaldi. There is a Treasury, which houses the Iron Crown of Lombardy and other important works. The interior is largely Baroque.