Kaltenburg Castle, Medieval castle ruins in Lonetal, Germany

Kaltenburg Castle is a ruined hilltop fortress on a rock spur about 60 metres above the confluence of the Hürbe and Lone, between Hürben and Burgberg. Its irregular pentagonal outer wall was flanked by two square and three round towers; the two tall square towers with pyramidal roofs, added in the 1677 rebuild, stand on the valley side. A 4-metre-high, 16-metre-long stretch of the 3.4-metre-thick shield wall survives from the 12th-century tower house.

Construction took place between 1150 and 1180, initiated by Heinrich von Kalden during the reign of Emperor Frederick Barbarossa. The fortress served to control and settle this hilly region during the High Middle Ages.

The castle's name is linked to Heinrich von Kalden, a marshal under Emperor Frederick Barbarossa, and construction is dated to 1150–1180. In 2015 the Interessengemeinschaft Kaltenburg e. V. acquired the ruin and has since carried out work such as closing a wall gap in March 2017 and securing the shield wall in 2018. The site now serves as an event venue, tested on 22 July 2020.

This site sits on a walking trail and can be reached on foot from the village of Hürben, with the path going through the Lone valley. Sturdy shoes are recommended since the climb to the ruin follows steep, unpaved terrain.

Right beside this castle site lies Charlottenhöhle, one of the longest cave systems in the Schwäbische Alb region. Visitors often combine both locations in a single trip, experiencing the ruins, woodland, and subterranean passages together.