Burg Rettenberg, Castle ruin and archaeological site in Rettenberg, Bavaria, Germany

Burg Rettenberg, also called Vorderburg, is a ruined tower castle on a roughly square rock head about 30 by 30 metres, on the northern ridge end of the Rottachberg at around 1020 metres, some 300 metres above the Illertal. Only the south and west walls survive: the windowless south wall is about 18 metres long, the west wall about 9 metres, with window openings, and the masonry still rises up to 9 metres.

The site was first documented in 1130 under the name Adelprecht de Rotinberch. Ownership passed from the Lords of Rettenberg to the Diocese of Augsburg around 1350.

The castle gave its name to the Lords of Rettenberg, the family that held the advocacies of several monasteries in the Oberallgäu. In 1525, during the German Peasants' War, the peasants of the Rettenberg bailiwick joined the Allgäu League, and gunpowder from the castle was used by the rebels. After their defeat at Leubas, the Swabian League stored the weapons of the defeated in Rettenberg and Fluhenstein castles.

The ruins sit about 600 meters northwest of Großdorf village. Access has been restricted since 2013 due to the risk of falling rocks from the unstable structure.

The ruin has never been included in the catalogue of the Burgenregion Allgäu, and unlike nearby Burg Langenegg, secured in a loop of the Iller at the end of the 20th century, no conservation work is visible on its walls. Only two high, heavily endangered stretches of masonry remain of one of the oldest tower castles in the Allgäu.