In 1926, alongside
Luigi Figini,
Guido Frette, Sebastiano Larco,
Gino Pollini,
Carlo Enrico Rava, and
Giuseppe Terragni, Castagnoli co-founded
Gruppo 7, a key initiator of
Italian Rationalism. He left the group in 1927 due to professional obligations and was replaced by
Adalberto Libera. In the following years, Castagnoli gained recognition through projects for competitions and exhibitions (notably the
5th Milan Triennial and the
Pirelli pavilion) and collaborated with
Piero Bottoni and
Antonio Cassi Ramelli. Between 1931 and 1932, he published a series of technical articles in
Rassegna di Architettura, focusing on innovative building materials. In 1935, he joined the Società Telefonica Interregionale Piemontese e Lombarda (
STIPEL), where he worked for over 25 years as director of real estate affairs, overseeing the planning and construction of offices, telephone buildings, switching stations, and air-raid shelters across
Lombardy,
Piedmont, and the
Aosta Valley. Notable projects include the
radio relay tower on Colle della Maddalena and the telephone headquarters on Via Confienza in
Turin. In 1946 and 1948, he was commissioned by president
Luigi Einaudi to restore his residences in Turin and
Dogliani; their correspondence is preserved in the Einaudi archives. ==References==