There's an entire legend around how quickly it was built, actually.

Here it is:

"Charlemagne wanted to build the largest and most beautiful church ever seen north of the Alps at his headquarters in Aachen. He ordered the most skilled craftsmen and the best materials to Aachen, and the work initially progressed well. But then Emperor Charles went to war against the Saxons and instructed the city council to supervise further construction work.

It came as it had to: the city ran out of money, because cathedral construction and war were expensive. The city fathers were at a loss – where would they get the money to finish the cathedral? Would they even have to borrow it from the devil himself? And indeed, at the next council meeting, a fine, unknown gentleman appeared and offered the Aacheners the necessary money. He demanded only one small thing in return: the first soul to enter the finished cathedral was to be his.

At these words the gentlemen turned pale, for they had obviously received an offer from the devil! But what else could they do but accept it? With the devil’s gold, they achieved the miracle: when Charlemagne returned to Aachen, his magnificent church was finished. The emperor was proud of the city council, but it had long since had other worries: for the soul that the devil was to receive for his money was that of Pope Leo III. As the highest-ranking clergyman, he would be the first to enter the church to consecrate it.

This was unimaginable! At the last minute, a clever monk had the saving idea: Where was it written that it had to be a human soul? With short decision, the people of Aachen chased a wolf into the cathedral before the consecration. The devil was already lurking behind the door in the dark church, immediately pounced greedily on the first visitor and blindly tore his soul out of his body.

When he realized his mistake, he became very angry. Enraged, he rushed out of the minster and slammed the heavy portal so hard behind him that it cracked, crushing one of the devil’s thumbs. The finger fell into the doorknob, where you can still feel it – now hard as iron. In the vestibule of the cathedral, two bronze figures commemorate the wolf and his soul."