Cathedral of Saint Domnius

Catholic cathedral in Split, Croatia, originally built c. 305 AD as the mausoleum of Roman Emperor Diocletian within his palace complex. The octagonal structure stands 24 metres high with walls 2.75 metres thick, reinforced with 28 granite and marble columns brought from Greek and Egyptian temples. In the 7th century, after refugees from destroyed Salona settled in the abandoned palace, Diocletian's sarcophagus and pagan statues were removed and the building was converted into a Christian cathedral by Bishop John of Ravenna. It retains its original Roman brick dome. Regarded as the oldest Catholic cathedral in the world still in use in its original structure. A Romanesque bell tower was added beginning in the 13th century.