On the exterior, the bell tower, standing directly over the transept crossing, is the most visible feature. It is divided into five tiers, of which the lower three, with Romanesque arches, date from the 12th century and the upper two from the 14th century. The spire was added in the 15th century. The bell tower is slightly inclined towards the west direction, which is why from certain standpoints the bell tower roof, whose axis is perpendicular to the ground, appears to be inclined to the tower itself.