Aachen was Charlemagne's favorite place to reside. His northern capital, where he built a palace and founded the cathedral that today contains his tomb.
Paris had been the capital of the barbarian Frankish king Clovis (466-511). Under the Charlemagne it was largely ignored and in decline. Charlemagne visited it no more than four times.
Venice was under the rule of Giovanni Galbaio and allied with Constantinople.
In Rome, on Christmas Day in CE 800, as Charlemagne knelt at St. Peter's, Pope Leo III placed a gold crown on his head and declared him emperor. This implied Roman emperor, and the Byzantines were outraged. They believed that the Roman Emperor resided in Constantinople and that Charlemagne was a barbarian. Pope Leo III saw the Christian church centered in Constantinople as a rival.
Barcelona was founded by the Romans 27-19 BCE. Muslim invaders captured the city without resistance from a mix of Visigoth and other Christians in 717. The cathedral there had been converted into a mosque and taxes levied on non-Muslims while religious freedom and civil government had been largely respected. Charlemagne's son, Louis the Pious, took the city in 801 after a siege of several months. (Charlemagne reigned to 814.)