Burg Langenegg, Medieval castle ruins in Martinszell, Germany.

Burg Langenegg is a castle ruin on a forested hilltop within a bend of the Iller River near Waltenhofen. The remaining stone tower with rectangular base shows the solid construction style of the medieval period.

The main tower was built around 1250. When the Lords of Langenegg died out in 1415, the castle passed through an heiress to the Lords of Rauns, who later sold part of it to the Kempten patrician Winter, who took the name von Langenegg. After the last lord died in 1647, it reverted to the Prince-Abbot of Kempten, who let it decay; from 1734 it served as the abbey's workhouse and poorhouse.

From 1734 the site served as the Kempten abbey's workhouse and poorhouse. In 1775 inmate Anna Schwegelin was denounced as a witch by a fellow prisoner after a quarrel; the abbey court sentenced her to death, but the sentence was not carried out. She died in Kempten prison in 1781 and is called the last witch of Germany.

The site can be reached via a small road from Thanners, and sturdy footwear is recommended for the narrow walking path. The hilltop setting provides room to explore and the surrounding forest offers shade.

The keep's walls are 1.3 to 2 metres thick and stand on a footprint of 12.8 by 14.6 metres. They are built of nagelfluh, a local conglomerate rock. The tower was structurally secured between 2001 and 2004.