The best look at ancient Constantinople is a church-turned-mosque that's been considered among the greatest houses of worship in both the Christian and Muslim worlds: Hagia Sophia, the Great Church of Constantinople. Built by the Byzantine Emperor Justinian in the early 6th century on the grandest scale possible, it was later converted into a mosque by the conquering Ottomans. Today it's a museum. Hagia Sophia, which marks the high point of Byzantine architecture, is the pinnacle of that society's sixth-century glory days.

This church was completed in 537, just about when Europe was entering its Dark Ages. For four centuries after that, Christians in Europe looked to Constantinople as the leading city in Christendom and this was its leading church.

This clever dome-upon-dome construction was the biggest dome anywhere until the cathedral of Florence was finished during the Renaissance 900 years later. The vast interior gives the impression of a golden weightless shell, gracefully disguising the massive overhead load supported by masterful Byzantine engineering. Forty arched windows shed a soft light on the interior, showing off the churches original marble and glittering mosaics.

But the Byzantine Empire collapsed in the 15th century, and Hagia Sophia was turned into a mosque. Christian mosaics were plastered over, and new religious symbols replaced the old.

This church was built to face Jerusalem; mosques faced Mecca. When Hagia Sophia became a mosque, they couldn’t move the church, but they could move the focal point of the praying. Notice how the prayer niche is just a bit off-center. That’s because it faces Mecca.