Excerpts from Books and Wikipedia
"It was important that the new cathedral be of epic proportions since it was to house the famous holy relics of the three kings who brought gifts to the infant Jesus, brought from Milan by the Holy Roman Emperor Frederick Barbarossa in 1142." [Furtado: 1001 Days] "In 1164, the Archbishop of Cologne, Rainald of Dassel, acquired the relics of the Three Kings which the Holy Roman Emperor, Frederick Barbarossa, had taken from the Basilica of Sant'Eustorgio, Milan, Italy. (Parts of the relics have since been returned to Milan.) The relics have great religious significance and drew pilgrims from all over Christendom. It was important to church officials that they be properly housed, and thus began a building program in the new style of Gothic architecture, based in particular on the French cathedral of Amiens." [Wikipedia]