Old Falkenstein Castle, Medieval fortress in Falkenstein/Harz, Germany.

Old Falkenstein Castle is a burgstall, the site of a high medieval hill castle on a rocky ridge at 335.6 m above the Selke valley. Its oval inner ward measured about 25 by 65 metres, with a narrow outer ward around 85 metres long; the whole site ran some 200 metres. Remnants of the ring moat, the advanced rampart and the northern ring wall remain visible.

Old Falkenstein Castle was probably built in the 11th century under Emperor Henry IV, with Benno II of Osnabrück as architect. After the Battle of Welfesholz on 11 February 1115, Duke Lothair of Süpplingenburg had it slighted; it was never rebuilt. Pottery shards suggest it was abandoned from the early 12th century, when nearby New Falkenstein Castle was built.

The name Falkenstein refers to the rocky ridge above the Selke valley where the castle stood; the neighbouring New Falkenstein Castle, about 1.8 km east-northeast, took over its role after 1115. Pottery shards found at the site indicate the castle was probably no longer occupied from the beginning of the 12th century. A cross on the ridge commemorates local history researcher and monument curator Hans Reißmann, who died here in an accident on 11 July 2003.

The site can be reached on foot from the town of Falkenstein via a marked path that is easy to follow. A visit is most enjoyable during warmer months when the paths are dry and views of the valley are at their best.

A metal cross on the rocky ridge marks where local historian and monument curator Hans Reißmann died in a fall on 11 July 2003. The castle itself survives only as a burgstall: sections of the ring ditch, the outer rampart and the northern ring wall remain visible. The whole site once stretched about 200 metres along the ridge.