The Reduit was built in 1830-32 by the Austrian engineer General Franz Scholl to reinforce the Kastel fortifications and as barracks for the Kastel bridgehead. The horseshoe-shaped, two-storey fort with a free-standing central building (réduit) on the Rhine side is adorned with a portal with mighty columns and a coat of arms trophy. Built on 1,800 oak piles directly on the banks of the Rhine in marshy gravel, the structure served the federal fortress of Mainz until 1866 to protect the ship bridge installed by the Elector of Mainz Johann Philipp von Schönborn, whose name the bastion built to protect the flanks of the complex still bears today. The inscription with the sentence "Cura confoederationis conditum MDCCCXXXII" ("Founded to protect the Confederation in 1832") points to the original purpose of the redoubt.

During the Second World War, the roof of the building and numerous superstructures were destroyed by bombs. In the 1960s, Kastel clubs moved into the Reduit.

Today it is home to the Kastel local history museum, Museum Castellum. In summer, the inner courtyard is used for open-air cinema, concerts and similar events.