During late La Tène period in the site of Birg was build a celtic oppidum.Then in 4th century A.D. the Birg was occupied by a roman fortification with a tower.
In the middle of 5th century A.D. the Birg remained under control of Romanized population while the Western Roman Empire collapsed.Finally the Birg became a medieval castle.
https://www.schmelz.de/fileadmin/Paulus/Freizeit_und_Tourismus/Wanderwege_Nodic_Walking/Birg.pdf