Burg Brennberg, Medieval castle ruin and observation tower in Brennberg, Germany.

Burg Brennberg is the ruin of a Ganerbenburg on a granite rock at 653 m above sea level, about 50 m above the village of Brennberg. In the 14th century the older castle was divided into two parts, Oberbrennberg and Unterbrennberg, linked by a central courtyard. The western part had a tower and the Münchstein well; the eastern part had its own tower. The site is a protected monument (D-3-75-120-1).

Wernher I de Prenberg built the first structure on the western hill around the early 11th century. In 1326 the line died out and Agnes de Prenberg married Friedrich I Auer, who expanded the castle from 1334 as his new seat. In 1379 his sons Friedrich II and Hans divided the estate into Oberbrennberg with the western tower and Münchstein well, and Unterbrennberg with the eastern tower, both linked by a central courtyard.

The name Brennberg comes from the three 'burning mountains' already shown in the 11th-century coat of arms of the de Prenberg family. The castle was split in 1379 between the brothers Friedrich II and Hans Auer into Oberbrennberg in the west and Unterbrennberg in the east. A grave monument to Achatz I. von Nußberg, who died in 1480, stands on the west side of the church tower of St. Rupert in Brennberg.

The ruin sits on a granite rock at 653 m, about 50 m above Brennberg and 23 km east of Regensburg; the climb is on foot only. In clear weather the tower view reaches the Alps. Nearby ruins include Falkenstein and Sengersberg to the northeast, Heilsberg to the south, Siegenstein to the northwest and Lobenstein to the north.

Burg Brennberg was split in 1379 between the brothers Friedrich II and Hans Auer: Friedrich took the tower and the Münchstein well in the west, Hans the tower in the east, creating Oberbrennberg and Unterbrennberg. The two halves stayed linked by a central courtyard, the mark of a Ganerbenburg held by co-heirs.