The castle squats on a porphyry spur of Mitterberg overlooking the confluence of the rivers Etsch and Eisack. Sigmundskron was always something special; it is one of the oldest castles in South Tyrol and, with its five-metre-thick walls, an early example of the art of fortified construction. 945 A.D. is the year of the earliest extant record of the castle, which at the time was called Formicaria (and later Formigar). In 1027 the Emperor Conrad II presented the castle to the Bishop of Trent. In the 12th century the castle passed to high-ranking civil servants, who thereupon called themselves “von Firmian”. Around 1473 Duke Sigmund the Wealthy, Prince of the Tyrol, bought the castle, had it converted into a fortress and changed its name to Sigmundskron. Only a few minor structures of the old Formigar Castle remained, mostly at the highest point of the grounds. Due to financial difficulties, however, Sigmund soon had to mortgage the castle, which subsequently fell into decay. At the end of the 18th century the castle belonged to the Earls of Wolkenstein, from 1807 to 1870 to the Earls of Sarnthein, and from then until 1994 to the Earls of Toggenburg.

The castle is a significant political symbol for the South Tyroleans. In 1957 the biggest demonstration in the history of South Tyrol took place there at the instigation of Silvius Magnago. More than 30,000 South Tyroleans gathered at the castle to protest the failure to implement the provisions of the Paris Agreement and to call for separate autonomy for South Tyrol (“Away from Trent”). The ruins of the castle were finally acquired by the Bozen provincial authority.