She was also involved in the C. G. Jung Institute, Zürich, of which she became head in 1928,[3] A 2007 review of Villa of Mysteries for the Spanish newspaper El Pais emphasized that it is "not a work of philology or history" but rather a "very Jungian" volume that seems to interpret not just a particular set of frescoes in a particular city but to expand its analysis to all of Roman culture on the understanding that what happened in Pompeii also happened elsewhere in the Empire.
Part of a "rich and colorful world of wealthy patrician houses and brothels," the Villa of the Mysteries was a place where women were initiated into "the secret – and complex – rites of Bacchus and Ariadne, under the sign of Orpheus."
Fierz-David examines the way in which this initiation process involves the workings of the subconscious and the awareness of "the hidden and terrifying side of God," and her interpretation, while "closely linked to Jungian psychoanalysis," continually brings to bear a "very rich knowledge of classical mythology.
His twin brother Heinrich-Karl Fierz also studied with Jung, trained at the Burghölzli Mental Hospital in Zurich under H. W. Maier and Manfred Bleuler, and wrote his doctoral dissertation on electroshock therapy.
The younger Heinrich worked as a psychiatrist at the Binswanger Clinic, Sanatorium Bellevue, and in 1964 co-founded the Klinik am Zürichberg, serving as its first medical director.