The largest and best-preserved fortified church in Germany is undoubtedly a defining feature of the pearl of the Streu. With five towers, six bastions, a double defensive wall and the mighty Renaissance church of St. Michael, it dominates the townscape of Ostheim in an impressive way.

The fortifications with numerous vaulted cellars and arcades were built as early as the 15th century. The latter are small residential and storage buildings that were erected inside the fortifications. A small Gothic St. Mary's Church stood in the middle of the complex. At the end of the 16th and beginning of the 17th century, the church of St. Michael was built in the Renaissance style. Since then, it has completed the overall picture of the famous Ostheim fortified church in its present form.