When the Romans expanded across Europe 2,000 years ago, they made inroads into almost every corner of the continent, fighting as far away as Scotland and sending its coins to what today is Estonia.

But Germany posed a particular challenge. In year 9 of our modern calendar system, the Romans suffered an embarrassing defeat in the Battle of the Teutoburg Forest near the modern day city of Hannover. They were permanently pushed back to the western side of the Rhine river.

Yet the Romans were quite active there and they left behind a vast number of architectural masterpieces. One of the most astonishing buildings from that era — the country’s oldest known public library — is only now being uncovered.

Built about 150 years after the Teutoburg Forest battle, its walls reemerged after centuries of darkness during the construction of a community center next to Cologne’s famous cathedral. When the walls were discovered last year, researchers assumed that they had come across a community hall. But an analysis found that the building was most likely used to store up to 20,000 scrolls of parchment.

The room was estimated to be 65 feet long and 30 feet wide, with a 30 foot high ceiling. But what really captured the researchers’ attention were the roughly 30-inch deep wall recesses, similar to other libraries during the Roman era. It may be the first such discovery of a library in the Roman Empire’s northwestern regions, which at its peak spanned France, Britain and western Germany.