Burg Altnußberg, Medieval castle ruin and observation tower in Geiersthal, Germany.

Burg Altnußberg is a castle ruin with an observation tower on Schlossberg hill in Geiersthal, featuring preserved stone walls, defensive structures, and a central tower. The tower is open to visitors and provides views of the surrounding countryside.

The Lords of Nussbach, ministeriales of the Counts of Bogen, were first mentioned in 1125 and gave their name to the castle built between 1174 and 1194. The family renamed itself Nussberg from 1194, and the castle itself was first documented in 1255. It passed to the Lords of Degenberg; in 1468–69, during the Böcklerkrieg uprising against Duke Albrecht IV, it was set on fire and razed after surrender by castellan Ratzko von Rayol.

The name Altnußberg comes from the Lords of Nussbach, first mentioned in 1125 as ministeriales of the Counts of Bogen. Around 1194 they renamed themselves Nussberg, likely after building the hilltop castle between 1174 and 1194. The castle itself was first documented in 1255. The museum displays finds from the 13th to 15th centuries, and the Geiersthal registry office offers weddings in the museum building.

The best time to visit is in good weather, when access to the ruin is safest and views from the observation platform are clearest. The castle is reached by walking paths, and a tavern on site offers refreshments during opening hours.

The five-sided bergfried, rebuilt in the 1980s, stands 22 metres high on a 7 by 7.5 metre base with 1.5-metre-thick walls. It now serves as an observation tower, and the nearby museum building displays finds from the excavations, including weapons, jewellery and household goods from the 13th to 15th centuries.