City and spa in Germany; northern capital of the empire of Charlemagne (6)

I believe the answer is:

aachen

(Other definitions for aachen that I've seen before include "German spa city" , "Aix-la-Chapelle's other name" , "German name for Aix-la-Chapelle" , "German city, Charlemagne's imperial capital" , "old capital city" .)

I've seen this clue in The Mirror.

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