Con­struc­tion of this pala­tine chapel, with its octag­o­nal basil­i­ca and cupo­la, began c. 790–800 under the Emper­or Charle­magne. Orig­i­nal­ly inspired by the church­es of the East­ern part of the Holy Roman Empire, it was splen­did­ly enlarged in the Mid­dle Ages.