Diez an der Lahn is a picturesque small historically significant town in Germany's Rhein-Lahn district in Rhineland-Palatinate, on the borders of Hesse. Diez is the administrative seat of the municipality of Diez. Sitting on the confluence of the Lahn and Aar rivers, the town and the area have been inhabited by humans since the Stone Age. The old town is dominated by an eleventh century castle, now a youth hostel and museum. It is the ancestral home of the House of Nassau-Dietz, which in 1815 became the Dutch Royal Family.