Castulus is the name of several early Christians:
1. The zetarius, or manager of the summer and winter dining-rooms of Diocletian or Maximiari. He lodged in a garret in the palace. and sheltered the Christians there. He was examined three times, then thrown into a pit, and finally buried alive. He is commemorated March 26, and a cemetery on the Via Lavicana is said to bear his name.
2. An Arian presbyter, whom Ambrose rescued out of the hands of the orthodox multitude at the time of the conflict about the basilicas at Milan (Ambrose, Epist. 20, 5).
3. A martyr along with Zoticus (q.v.) in Egypt or in Africa, Jan. 12.
4. A martyr at Ancyra, in Galatia, Jan. 23, according to the Hieronymian Mart., Smith, Dict. of Christ. Biog. s.v.