It was built from east to west. This is typical for church buildings. If you look at the arcades between the central nave and the side naves, you will see that they become more and more pointed towards the west, i.e. in the direction of construction: The arches become more and more gothic.
You can also follow this development up in the vault: The eastern bays are still formed by Romanesque groined vaults. There an extremely massive wall absorbs the ceiling load. On the opposite side of the cathedral the situation is different: the western yoke and the western choir are already spanned by Gothic ribbed vaults.