More than 2500 years ago, far after the settlement of this land by the people of the Bandkeramiker about 7000 years ago the Celts settled permanently in the Gleichberg area.

They built hilltop castles on both Gleichberge mountains. Located in the southern foothills of the Thuringian Forest and the Rhön, these were part of a system of Celtic hilltop settlements with the function of the northernmost posts before the Germanic tribes advancing from northern Germany. On the small Gleichberg, also called Steinsburg, the Celts built a castle complex, which in its largest expansion phase covered about 68 hectares and provided a home for 2 to 3 thousand people. At that time a very large and widely important settlement, located at the crossroads of two important inner-European trade routes.

It is believed that this was the ancient city of Bicourgion which the Greek historian Claudios Ptolemy describes about 100 AD in his geography of Germania Magna. Thus, the stone castle near Römhild would be the oldest settlement in Thuringia.

The Oppidum Steinsburg is the largest archaeological monument in Germany north of the Main River.

From the high plateau of Steinsburg Castle, which rises far above the landscape, there are views of the Rhön Mountains to Kreuzberg, to the German Castle Museum on neighboring Veste Heldburg, to Upper Franconia to Veste Coburg and to the Thuringian Forest.