The ancient fortress that served as the capital of the Byzantine Empire is the Hagia Sophia, located in Constantinople, modern-day Istanbul, Turkey. Originally built as a cathedral in the 6th century during the reign of Emperor Justinian I, it is renowned for its massive dome and stunning mosaics. The Hagia Sophia symbolized the power and grandeur of the Byzantine Empire, blending architectural innovation with religious significance. Following the Ottoman conquest of Constantinople in 1453, the structure was converted into a mosque, reflecting the city's new Islamic identity, and it became a significant monument representing both Christian and Islamic heritage.