Ostheim Fortress Church is a fortress church located in the Lower Franconian town of Ostheim vor der Rhön in the Rhön-Grabfeld district. The church building of St. Michael there is located within a double ring wall built between 1400 and 1450 with a zwinger in between. The double ring wall has five defense towers and is reinforced with six bastions halfway along the wall. The Protestant church, built in the Renaissance style on the foundations of a predecessor church, dates from 1615 to 1619. Within the fortifications there are 66 vaulted cellars with 72 lofts, which served as protective dwellings in times of war and in which the local inhabitants kept their belongings safe in times of crisis. With a floor area of 75 by 75 meters, it is considered the largest and best-preserved fortress church in Germany. Some of the vaulted cellars are used by the local population as storage cellars.