Saint Castulus was a recent convert to Christianity during the Diocletian persecutions. When the persecutions began, he served as the champberlain of Emperor Diocletian and was the husband of Saint Irene of Rome. He helped Christians find shelter by bringing them to his home and keeping them hidden away from the Roman guards who were actively arresting and killing Christians. Secretly, he began arraigning Christian services inside of the Emperor's palace. As the persecution continued and worsened, he began bringing converts alongside Saint Tiburtius to Saint Pope Caius for baptism.
One night, while bringing another group of converts to the Pope, he was betrayed by an apostate by the name of Torquatus and arrested. He was pulled in front of the prefect, Fabian and ordered to recant his Christian faith. He stood in chains and announced he was Christian proudly, knowing it would almost certainly carry a painful sentence of death. Saint Castulus was tortured by being burned alive in a sandpit on the Via Labicana. Saint Irene buried the body of her husband before also caring for another soon to be martyr - Saint Sebastian. Saint Irene was martyred as well after burying Saint Sebastian's body.
A church was built on the site of his martyrdom sometime in the six hundreds and his relics were taken first to Bavaria in 1171 then to Landshut in 1604. Today his relics rest in the church of Saint Martin's and Kostel Sv Hastala in Prague. His feast day is celebrated on March 26th.