Castra Martis (Bulgarian: Кастра Мартис) was a Roman fortified garrison (castra) in Dacia which became a town and bishopric and remains a Latin Catholic titular see. Castra Martis, named after the Roman god of war Mars, on the modern site of Kula (Latin/Italian Cula), in Vidin Province in northwestern Bulgaria, served to protect the road through Vrashka Chuka pass in the western Balkan mountains. It was important enough in the Roman province of Dacia ripensis to become a suffragan of the provincial capital’s Metropolitan Archdiocese of Ratiaria, in the sway of the Patriarchate of Constantinople. Its only recorded Suffragan Bishop was Calvus, participant at the Council of Serdica in 343 (called by the Pope, boycotted by most Eastern sees).
Source: Wikipedia
