Its ruins are situated on a wooded hilltop west of Pusarnitz in the municipality of Lurnfeld near Spittal an der Drau.

Feldsberg has an irregular quadrangular ground plan, following the terrain of the hill plateau.

On the north and west side there are walls, still up to 8 metres high, with herringbone masonry (opus spicatum).

Feldsberg Castle was a bone of contention in disputes between the Counts of Gorizia and the Archbishopric of Salzburg.

The castle may have been in a state of decay since the end of the 16th century, after jurisdiction had been transferred from Feldsberg to Sachsenburg.