A cathedral and a bishop's palace existed here already within the 9th century. The construction of the cathedral of today started in 1196, about 150 years later, the church may have been completed, but a massive addition of the cathedral was planned in 1339.

It would have more than doubled the size of the structure by means of an entirely new nave and two aisles ranged perpendicular to the existing nave. The planned cathedral would have been larger than (old) Saint Peter in Rome, it was never completed.

The construction was halted by the Black Death in 1348. Then errors in the static of the building got evident.

A kind of under-church, that was underneath the choir of the cathedral had to be filled up, to give stability... and got completely forgotten.

In 1999 this crypt was found and got excavated over the next years. Some some phantastic frescoes from around 1280 were discovered here.

When the crypt about 700 years was filled up with debris, to stabilize the church above, it was clear that it would be unseen and - gone "for ever". So the workers had no hesitation, to leave graffiti on the walls.