From 795 to 843, Aachen, in western Germany, was the capital of the barbarian kingdom of Francia, also known as the Kingdom of the Franks. The Franks were Germanic people who ruled large parts of Western Europe from late antiquity to the early Middle Ages.  In the late eighth century, the king of the Franks and king of the Lombards—Charlemagne—commissioned a palace chapel to be built in Aachen. (Charlemagne, also known as Charles the Great, went on to become the Holy Roman Emperor in 800.)  The chapel embraces the classical and Byzantine traditions and was the first vaulted structure north of the Alps since antiquity. It inspired religious architecture from the Carolingian until the early medieval period. Constructed between 793 and 813, the octagonal Palatine Chapel features two stories, the lower of which has eight pillars supporting arches, and the upper story is adorned with eight Carolingian bronze gates.  Originally, the chapel …