Not only is Trier Germany’s oldest city, but during the third century it served, along with Rome, Alexandria and Constantinople, as one of the capitals of the world. Constantine the Great began ruling there in 306 C.E., after which the city grew in importance. During his reign of 31 years​—he is said to have resided in Trier until 312—​it experienced a tremendous building program.

Diocletian abdicated in 305 C.E., and the next year Constantine the Great took up residence in Trier.”

Ahead of us is an imposing brick building at least 30 m (98 feet) high, its walls full of large arched windows. Peter is already explaining: “ . . . the basilica, part of Constantine the Great’s imperial palace, where he, sitting under a canopy, received guests for his festive and political gatherings.