Built in its first incarnation in the 13th century, the medieval clock tower has served as a gate tower, prison for women convicted of sexual relations with clerics, master timekeeper, center of urban life and now tourist attraction. The famous astronomical calendar clock was not installed until the 15th century and is one of the oldest in Switzerland. Two bronze bells - one of which is the great hour bell cast by Johann Reber and unchanged since 1405 - are struck on the hour by a gilded mechanical figure of Chronus.