Fautsburg, Castle ruin in Bad Wildbad, Germany.

Fautsburg is a castle ruin above the Rehmühle in the Aichelberg district of Bad Wildbad, in Baden-Württemberg's Calw district. Of the hillside castle, only the bergfried, rebuilt in the early 1960s as a viewing tower and roofed in 2007, and some wall remains survive. The tower measures 7.15 by 6.7 metres with walls about two metres thick and reaches roughly 20 metres with its roof; a wooden staircase of 33 steps leads to a platform 14.3 metres up.

Fautsburg is first documented on 23 February 1276, when it was held by the Counts of Calw; it may have been built by the Counts of Hohenberg around 1200. In 1323 the Lords of Hornberg sold half to Count Eberhard of Württemberg for 300 pounds heller, and the other half later reached Württemberg through Count Palatine Rudolf IV of Tübingen. In 1561 Duke Christoph enfeoffed the reformer Johannes Brenz with the castle, and in 1570 Duke Ludwig took it back and gave it to Hirsau Abbey. By 1604 a forest servant lived in the already decaying ruin, whose walls were later quarried by the Rehmüller family.

The ruins of Fautsburg demonstrate medieval construction methods through exposed sections of walls, revealing details about architectural practices from that period.

The ruin is accessible via marked hiking trails starting from Bad Wildbad, with informational signs along the way. Sturdy walking shoes and time for a forest hike are recommended, as the paths can be steep and wind through wooded terrain.

From its elevated position, visitors can enjoy views of the surrounding valleys, showing how the original castle used its strategic location. This placement allowed the inhabitants to control the landscape below and defend against attacks from multiple directions.