"In the 5th century A.D. the Roman Empire, which had ruled across Europe for 600 years, was facing collapse. More than 100 years earlier in 332 its Emperor Constantine, who'd erected his victory arch in Rome, had moved the capital of the empire to a city in modern day Turkey which was named Constantinople.

Constantine was the first Roman emperor to become a Christian and over the next 1000 years his Eastern Roman Empire embraced Christianity and became the Byzantine Empire.

Constantine changed the course of human history. With his adoption of Christianity, Christianity went from a persecuted minority sect to being the official religion of the Roman Empire.

Later, in the final years of the 1th century, Christian forces were more aggressive as Crusaders fought to retake Christendom's holy lands and sites from another younger religion on the march, Islam.

Meanwhile, Europe's great new sea faring and trading power, Venice, sought accommodation and profit from all sides, whether Christian, Jew, or Muslim, in a complex game of shifting alliances, hostility, and war.

In this episode of Empire Builders we explore 10 sites that made history. We visit Bryzantium's greatest church, the Haghia Sophia in modern day Istanbul, the Crusaders' greatest fortress,Crac de Chevalier in modern day Syria, and St Marks Basilica, in Venice, a showcase of the Venetian Empire at its peak.,Together through these great sites, and others we chart the rise and fall of the great Christian empires of Byzantium, the Crusades and Venice.