Vianden is situated at the southern border of the Ardennes, in a narrow valley, orientated from North to South. In roman times the crossing of the river Sûre / Sauer was made possible by a ford. This strategically favourable position was guarded by an early watch tower placed at the later castle’s location: on a longitudinally stretched ledge at half height between valley and high tableland. That ridge of rocks is part of the schist formed during the period of lower Devon – aged about 380 to 410 million years.
 

That metamorphic rock composed of layers of different minerals easily splits into thin irregular plates that supply the base construction material for the medieval castle and is still the main characteristic of its today’s visual appearance.