What is byzantium called now?

Byzantium was redeveloped by Constantine the Great who designated it as imperial capital and renamed it Constantinople after himself (it means City of Constantine). It was inaugurated in 330. After the Ottoman Turks took Constantinople in 1453, under the sultans, it was called Kostantiniyye and Islambol or Islambul. This turned into Istanbul when Turkey adapted the Latin alphabet in 1928. In that year, the new republican government also made it the city sole name.