Remains of a Celtic fort, Otzenhausen
The Celtic circular fort at Otzenhausen is one of the biggest fortifications the Celts ever constructed... In times of war, the circular rampart was a strong fortification against enemies. However, theories suggest this fort might have been more than just a refuge. It might, in fact, have been a permanent settlement, a village or the seat of a local leader...The first fortification was constructed in the 5th or 4th century BC, but the real heyday of construction dates to the 2nd and 1st century BC. For reasons yet unknown, the fort was abandoned shortly after this expansion.
— Wikipedia (slighltly redacted)
Saardom - Saarland's “Cathedral”
The state of Saarland has no offical cathedral; in the Catholic church, it belongs to the diocese of Trier, in the state of Rheinland-Pfalz. However, the small town of Dillingen has Saarland's tallest church, earning it the “cathedral” nickname.