Burg Fluhenstein, Medieval castle ruins in Sonthofen, Germany.

Burg Fluhenstein is a medieval castle whose ruins sit on elevated ground near Sonthofen, overlooking the Allgäu valley. Sections of stone walls and foundations remain visible, showing how the defensive structure was once organized and built.

In 1362 Oswald von Heimenhofen acquired the lordship of Berghofen from Kempten Abbey and moved his seat to a rock spur on the Walten. His line fell into debt: Erkinger sold his share to Augsburg's bishop in 1440 for 9,225 guilders, and Jörg the Elder sold the castle to the Hochstift Augsburg in 1477. Augsburg officials held it until 1769; after secularisation it was sold for demolition in 1808 and remains private property.

The castle chapel of St. Alexius was consecrated in 1501, when the Augsburg bishopric extended the complex with a north wing around 1500. The Heimenhofen family, who built the castle, later lost their estates: Jörg von Heimenhofen was called only Jörg Heimenhofer, and his brother Kleinhans married the serf Anna Mutzin, so their children fell into serfdom.

The ruin stands at the southern edge of Berghofen, a district of Sonthofen, on a slope of the Walten ridge. It is in a desolate state and can only be viewed from the outside; the interior is not accessible. The site is privately owned, so visitors should stay on the outside and leave the walls untouched.

The castle was sold for demolition to a farmer in 1808 after the secularisation of the Augsburg prince-bishopric, and the ruin has remained in private ownership ever since. Its position was militarily poor: a tongue of land above it allowed attackers to fire down into the site.