> Or did they followed some method that is today lost, possibly based on the position of a start or a constellation, resulting in an alignment on the Mercator p

> Or did they followed some method that is today lost, possibly based on the position of a start or a constellation, resulting in an alignment on the Mercator projection? I would not say that the method is today lost; while not aligned on a geodesic, the cathedrals are aligned on a spiral starting at the North Pole and ending at the South Pole which always travels at a constant bearing. It’s totally possible to recreate a line like this with a compass by walking such that the compass has a constant reading. Doing so over the Mediterranean, multiple mountain ranges, and bits of the Atlantic would be a bit difficult, though.