There’s nothing modest about the religious architecture of Istanbul.

In fact, the best way to appreciate the magnificence of ancient Constantinople is to visit a church-turned-mosque that’s considered one of the greatest houses of worship in both the Christian and Muslim worlds: Hagia Sophia, the Great Church of Constantinople. Originally built as a church by the Byzantine Emperor Justinian in the early sixth century on the grandest scale possible, it was later converted into a mosque by the conquering Ottomans. Hagia Sophia, which marks the high point of Byzantine architecture, is the pinnacle of that society’s glory days.

This magnificent church was completed in 537, just about when Europe was falling into the Middle Ages. And for four centuries after that, people in Europe looked to Constantinople as the leading city in Christendom, and this was the leading church.

This clever dome-upon-dome construction was the biggest dome anywhere until the cathedral of Florence was built 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 ingenious Byzantine engineering. Arched windows shed a soft light on the interior, showing off the church’s original marble and glittering mosaic work.

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.

Rick: So it’s a church turned into a mosque.

Lale: Yes. And churches are built to face Jerusalem; mosques, Mecca. Since they could not move the building, they moved the focal point of prayer towards Mecca. See the niche? It’s off the center towards right.

Rick: Facing Mecca.

Lale: Facing Mecca.

Rick: Very economic.

Today, while Hagia Sophia remains an important place of worship, tourists are welcomed up to the mezzanine level to marvel at the art, to ponder the tangled history of this venerable spot, and perhaps to give yourself the opportunity to be inspired by one of the most esteemed places in Islam.

While considered a must-see by visitors, it’s important for tourists to remember that Hagia Sophia remains a functioning mosque.