An envoy carrying a petition to Charlemagne would travel to his main residence and centre of government. Which place was that?

Explanation

Charlemagne travelled constantly, but he built a palace complex in the Rhineland and made it the fixed home of his household, his chapel, his records and his officials, so petitioners went there to find them. Rome is the misreading to watch for, because he was crowned emperor there, yet that was a single visit and the city was never the place from which he governed.