The Tour des Sorciers — Witches' Tower — is a medieval tower in the lower town that served for centuries as a prison, including during the waves of witch trials that convulsed Valais in the 15th and 16th centuries. Valais was, by some accounts, one of the earliest regions in Europe to prosecute witchcraft systematically; the trials here predate the famous later European witch hunts by several decades.
The tower is well preserved and forms part of the surviving medieval fortification of the lower town. It's not a formal museum, but it can be viewed from outside and is an atmospheric reminder of the darker uses to which medieval civic architecture was put. The name has stuck even though the tower was used for many categories of prisoner.
story: "Valais may have been the birthplace of the European witch hunt: trials here as early as 1428 used the full apparatus of systematic accusation, torture, and execution that would later spread across the continent. The numbers were staggering for the time — hundreds tried in a region of only a few thousand people."