It has been a guard tower, and a prison for women convicted of having sex with priests, but since the 15th century, it's been a clock tower with an elaborate astronomical clock. Hourly throughout the day, it puts on a great display of early animatronics. The show starts a few minutes before the hour with a little song and some drumming by a jester on top. On the hour, bears and an old bearded king get into the act. As well as the time, the clock shows the month, day, sign of the zodiac and phase of the moon. There are guided tours inside the tower that will let you have a look at the clockwork while the show is displayed outside. It can be booked at the tourist office and is definitely worth it if you love mechanics.