Hagia Sophia is one of the most beautiful historical building in the Istanbul. It was constructed as a Byzantine Orthodox Cathedral between 532 - 537 AD by the order of Emperor Justinian I. Hagia Sophia had been used as an Orthodox Cathedral until 1204 AD. It was converted into a Roman Catholic Cathedral in the Latin Empire period between 1204 and 1261 AD. In 1453 AD, Constantinople was conquered by the Ottoman Empire. Therefore, Hagia Sophia was converted into a mosque.