Blankenstein Castle, Medieval castle ruins in Gladenbach, Germany.

Burg Blankenstein is a castle ruin on a hillside near Gladenbach where stone walls and foundation remains still outline the original fortress layout. The surviving wall fragments show what once protected this strategic location.

Built before 1237 to guard the Cologne–Leipzig road, the castle was taken by Landgravine Sophie of Hesse in 1248 and destroyed to secure the inheritance of her underage son Henry. She had it rebuilt from 1255 and enfeoffed Gottfried von Rotenstein and Senand von Buseck. Archbishop Ruprecht of Cologne was held prisoner here from 1478 until his death in 1480; the castle burned in 1647 and a new administrative building rose in the ruins in 1648.

The castle's name comes from the blanken Stein, the bare rock of its hilltop site above Gladenbach. From 1255 its garrison was charged, together with the crews of Wallenfels and Neu-Dernbach, with guarding the Brabanter Straße and its crossing with the Westfalenweg and Herborner Hohe Straße near the Wilhelmsteine. After 1647 the ruins held a new Amtshaus built in 1648, seat of the Amt Blankenstein.

The site is reachable by marked hiking trails from a nearby parking area, and informational panels explain the layout. The grounds are easy to walk, though trail conditions vary with the season.

The castle's garrison also guarded the crossing of the Brabant Road with the Westfalenweg and the Herborner Hohe Straße near the Wilhelmsteine in the Schelderwald, together with the crews of Wallenfels and Neu-Dernbach. From 1478 to 1480 the Cologne archbishop Ruprecht von der Pfalz was held prisoner here.