Farnsburg was built in 1330 by the Lords of Thierstein and later acquired by the Habsburg barons of Falkenstein [de]. The barons went on to fight in the war against the Swiss, resulting in a siege of the castle in 1444.
The castle, which was still in good condition, changed ownership several times over the centuries until 1709, when the bailiff in residence at the time was expelled and the castle set on fire. Afterwards, the castle stones were used to build new castles elsewhere and Farnsburg fell quickly into ruin.
^"Kantonsliste A-Objekte:SG". KGS Inventar (in German). Federal Office of Civil Protection. 2009. Archived from the original on 28 June 2010. Retrieved 5 December 2017.
