Bentheim Castle sits on a sandstone outcrop above the town and is one of the best-preserved medieval fortifications in northwest Germany. The structure dates back to at least the eleventh century, though much of what you see today is from later rebuilding, and the whole complex was extensively restored in the nineteenth century by its princely owners. The views from the walls across the flat heathland toward the Dutch border are excellent, particularly in clear weather.
Inside the castle there is a museum covering the region's history, from its early medieval lords to the county of Bentheim's long political entanglements with both the Dutch Republic and later German states. The castle chapel, one of the oldest parts of the structure, is still used for occasional services. Allow a couple of hours to explore the full complex.