Hagia Sofia, Turkey
The oldest entry on this list, the Hagia Sofia, was completed in 537 AD under Emperor Justinian I. Not only is it one of the most iconic Byzantine structures and one of the most impressive architectural achievements of the time, but also served as the site where the Byzantine Emperors were crowned until the Ottomans conquered Constantinople in 1453. The circular, multi-coloured marble floor known as The Omphalion, "little navel", became the sacred spot for imperial coronations for almost a thousand years! With a long and varied religious history, its design has inspired many Orthodox churches and Ottoman mosques throughout the centuries. Today, this UNESCO World Heritage site serves as the Hagia Sofia Grand Mosque.