The Sonderhäuser Mikveh was discovered by chance in 1996 during archaeological excavations in the run-up to the development of the Postplatz square for the construction of a shopping center. It is directly adjacent to the remains of the medieval town wall and is fed with water from the nearby River Bebra. The mikvah is one of the oldest surviving Jewish ritual baths in central and northern Germany and is associated with the medieval Jewish population of the town of Sondershausen. With the Jewish pogroms in 1349, which also killed or expelled the Sondershausen Jews, it fell into disrepair and was forgotten for a long time.

The castle museum manages visitor inquiries and, in addition to the mikvah, also has a small exhibition on Jewish life and the later synagogue community in Sondershausen.