The bell tower grew in several stages. The lower, rectangular tower was the original tower. The octagonal upper tower was added in the 19th Century. The lower tower is flanked by two round towers and eight pillars. To the north and south, it is supported by buttresses that rise above the northern and southern main portals. The west face of the tower rises above a gallery above the central, west portal. The east face is supported by the central nave. The decoration on the western face of the tower has changed several times over the centuries. The lower, western window is quite short but wide.