On January 2 , 1945 , the medieval city center was bombed by the Royal Air Force and the U.S. Air Force and about ninety percent of it was destroyed in only one hour , with 1800 residents killed and roughly 100,000 left homeless . On January 2 , 1945 , the medieval city center was systematically bombed by the Royal Air Force and the U.S. Army Air Forces and about ninety percent of it was destroyed in only one hour , with 1,800 residents killed and roughly 100,000 displaced .