The Vikings came, raided – and stayed. Over the winter, at least. Instead of sailing home after every raid, they settled into fortified camps on the Rhine and Meuse and even made themselves comfortable in the emperors’ palaces.
In the autumn of 880 the Northmen took up winter quarters in Charlemagne’s palace at Nijmegen (the Valkhof). In spring they moved on – and burned the complex down. That is why almost nothing of Charlemagne’s palace at Nijmegen survives to this day.
After his return from Italy, Emperor Charles III gathered a great army in 882 and besieged the fortified Viking camp at Asselt on the Meuse. But instead of storming it, he negotiated. The Annals of Mainz even claim that a bribed adviser (Liutward of Vercelli) talked the emperor into peace just as the camp was about to fall. The leader Godfrid had himself baptised, received Frisia as a fief and the Frankish princess Gisela as his wife; the rest were paid off with silver. Charles’s reputation as a man who “bought his way out” was born.
The peace did not last long: in 885 Godfrid was lured into a trap over a conspiracy and slain.
The turning point came from the east of the empire. In 891 a Viking army had entrenched itself in a camp on the Dyle near Leuven (in present-day Belgium). King Arnulf of Carinthia had the fortification stormed and drove the Northmen into the river, where many drowned. Two “kings” fell, and 16 banners were captured. The great mainland raiding wave was broken.
How the Vikings reached Aachen, you can read in the post 881 – when the Vikings stood before Aachen.