According to tradition, he was the chamberlain (or officer, valet) of Emperor Diocletian and the husband of Irene of Rome.

With his friend Tiburtius, he converted many men and woman to Christianity and brought them to Pope Caius to be baptized.

[4] He was betrayed by an apostate named Torquatus and taken before Fabian, prefect of the city.

[4] Castulus was tortured and executed by being buried alive in a sand pit on the Via Labicana.

A church dedicated to Castulus at Rome, built on the site of his martyrdom, existed from at least the seventh century.