Farnsburg Castle, Medieval castle ruin in Ormalingen, Switzerland.

Farnsburg is a castle ruin in Switzerland that sits on a hilltop with partially standing stone walls from the Middle Ages. The remaining walls still show the original layout of different building sections and fortification structures.

Farnsburg was built in 1330 by the Lords of Thierstein and later acquired by the Habsburg barons of Falkenstein, who fought in the war against the Swiss, leading to a siege in 1444. The castle changed ownership several times until 1709, when the bailiff was expelled and the castle set on fire. Afterwards its stones were reused elsewhere and the site fell into ruin.

The castle shows how nobility lived in this region and how they defended their holdings. Visitors can still see how rooms were arranged and what structures were needed for daily life.

The ruin sits at 734 m on a rocky spur above the Hofgut Farnsburg, from where the Burgweg climbs to the site; marked paths also lead up from the Buuseregg bus stop. A modern spiral staircase inside the shield wall gives access to the viewing point, and a large plan on site explains the layout. The neck ditch survives only at the access bridge.

After an early 18th-century fire, many of the building's stones were removed and used to construct other houses. This explains why the ruin appears so fragmented today and how medieval building materials were recycled throughout the region.