The predecessor of St. Dominus's Cathedral was the Mausoleum of Diocletian. Fearing the foundation of Christianity's sway, he carried out a campaign of persecution of Christians during his tenure, but after death was counter-punched by Christians, destroyed the corpses, and the tomb was converted into a church. It is the greatest irony to him. The church's existing 13th century altar, as well as the sarcophagus statue, is of great cultural value.