The Vikings took over Valkhof hill in Nijmegen in 880 and wintered over in Charlemagne's palace there or so goes the standard narrative. One source tells that they did so again the following year.
The "Keizer" or "Caesar" Emperor was Charlemagne himself, Karel de grote, in Dutch; Charles the Great. His rule, the Holy Roman Empire, was a supposed re-institution of the original Roman empire after four centuries. Nijmegen had been an important city on the frontier at the northern Continental extent of the real Roman empire, and Charlemagne had supposedly accordingly maintained an important presence there during his rule.
The Keizer's palace in Nijmegen in written history was an important outpost here in an important city within that new, "Holy Roman" empire. Charlemagne would stop here on his trips through the region, according to that tradition. Nijmegen is proud of this legacy one of its various laudatory epithets is "Keizerstad," or "Emperor's town."
During the real Roman Empire, Nijmegen was a great market city and garrison.
The Emperor, "successor" to the great Roman imperial title, had ostensibly found Nijmegen an important symbol with its Roman pedigree, and had graced it with his occasional presence, and a palace.
The problem with the Viking story (that they overwintered at the palace) is the problem with the palace there's no evidence of it. More pointedly, there is no evidence of Carolingian occupation. (Carolingian being the adjective meaning "of the Charles, or Karl [the Great, or Charlemagne,] civilization.")
Albert Delahaye, archivist for the city of Nijmegen from 1946 to 1957 noted the lack of Carolingian artifacts in the city's archeology. He developed a few disenting ideas about Dutch history, besides. Delahaye is reviled and revered in Dutch letters more so the former, and especially by established historians but he raises an interesting point here.
The rebuilding after the accidental destruction of Nijmegen by Anglo-American bombers in WWII naturally involved extensive archeological work. Carolingian artifacts remained lacking.
It's possible that the story about Vikings wintering in Nijmegen is untrue.