Burg Forstenberg, Medieval castle ruins in Regenstauf, Germany.

Burg Forstenberg is a castle ruin in Regenstauf built around a pentagonal stone keep. The structure rises on elevated ground and shows the defensive design typical of noble residences from the medieval period in this region.

The castle was built in the 13th century as the seat of the Forstenberg lords and stood as a symbol of their authority in the area. Ownership changed around 1330, after which the fortress gradually declined in importance and was eventually abandoned.

The name Forstenberg comes from the ministerialis family of the same name, who probably built the castle in the 13th century as their ancestral seat; they served the lords of Hohenfels. After 1330 the castle passed to Friedrich der Hofer, who then called himself "von Vorchtenberg", and in 1387 a Winhir der Awaer is recorded as Pfleger of Vorchtenberkch. The nearby Schloss Karlstein, expanded from 1460 under Heinrich Zenger, drew the lords away, and by the 17th century the castle was abandoned.

The ruin stands at 425 m above sea level near Karlstein, a district of the market town Regenstauf, and is freely accessible at any time. The keep is separated from the larger outer bailey by a neck ditch about five metres deep; remains of ramparts and a flattened rectangular ditch lie in the outer ward.

The five-sided keep was built of rubble masonry with corner quoins, and a neck ditch about five metres deep separates it from the larger outer bailey. In that bailey, rampart remnants and a flattened rectangular ditch are still visible.