St Peter's, Trier

According to certain sources, the cathedral was commissioned by Emperor Constantine the Great and built on top of a palace of Saint Helen, his mother. Following the conversion of Constantine to Christianity, bishop Maximin (329-346) is said to have coordinated the construction of a cathedral, which at the time was the grandest ensemble of ecclesiastical structures in the West outside Rome. On a groundplan four times the size of the present cathedral no less than four basilicas, a baptistery and outbuildings were constructed. Archaeological research confirms that the current cathedral, as well as the adjacent cloisters and Church of Our Lady, is raised upon the foundations of ancient Roman buildings of Augusta Treverorum. The four piers of the crossing of the present church, as well as parts of the brick outer walls are remnants from this period. The fourth-century church was left in ruins by the Franks but rebuilt. It was destroyed again by the Vikings in 882. Under Archbishop Egbert (d. 993) rebuilding started, completed by Poppo of Babenberg (1016-1041). The famous west fa├žade dates from this period, although the apse was not finished until 1196. Throughout the centuries the church continued to be rebuilt and embellished, according to the fashion of the period with Gothic vaults, Renaissance sculptures and Baroque chapels, but the overall style of the building remains Romanesque with a Roman core.

https://www.en.dominformation.de/the-structure/history-of-the-structure-and-its-art/