Burgruine Kirnstein, Medieval castle ruins in Flintsbach, Germany

Burgruine Kirnstein is a castle ruin in Flintsbach built from stone with a rectangular layout. The remaining wall sections rise several meters high and dominate the surrounding valley landscape.

The fortress was built in the 12th century by the Lords of Neuburg-Falkenstein to control an important trade route through the Inn Valley. Following the decline of that noble family, the site passed to other Bavarian rulers.

The castle controlled the old road through the narrow Inn Valley, and around 1130 a "Leoprant de Churne" appears as a witness for Count Wolfger von Falkenstein. In 1568 the cartographer Philipp Apian marked it in his Bairische Landtafeln as "arx in monte sita", a castle set in the mountains; by 1585 Wiguleus Hund called it a "zerprochenes Schloß", a ruined castle.

The ruin can be viewed from a distance but requires caution due to its elevated position on a rocky outcrop. Visitors should wear sturdy footwear and be especially careful in wet weather conditions.

The west wall of the ruin collapsed in a storm in 2002. The surviving square enclosure, about 18 by 16 metres, still rises roughly eight metres, and much of its masonry dates from the rebuilding after the fire around 1430.