On top of the Kleine Gleichberg(the smaller of the two mountains) are the remains of a Celtic settlement, dating from about 100b.c. It's about a 45 minute walk from the parking lot to the top of the mountain. There are signs in german explaining the various sites along the trail. On a clear day, the top of the mountain offers an impressive view of the entire Thueringer Forest. The nearby Steinsburg museum doesn't have any English material per se, but the friendly staff gave me a photocopy of a magazine article in English explaining the site.
This museum, located in between two mountains, the "Gleichberge," is a display of early human settlement in Thueringen. The museum's main focus is on the early Celtic settlement on top of the Kleine Gleichberg, but also includes artifacts from Roman and Germanic settlements. Jewelry, tools, weapons, and other artifacts excavated from the area are on display.