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. Hagia Sophia, which marks the high point of Byzantine architecture, is the pinnacle of that society's sixth-century glory days. For four centuries after that, Christians in Europe looked to Constantinople as the leading city in Christendom and this was its leading church. But the Byzantine Empire collapsed in the 15th century, and Hagia Sophia was turned into a mosque. When Hagia Sophia became a mosque, they couldn’t move the church, but they could move the focal point of the praying.