There's also the medieval Holy Roman Empire which was officially a continuation of the Western Roman Empire, and at odds with the Byzantine Empire because they considered themselves the only legitimate Roman Empire, and these are all related but clearly different cultures.
And then there's the vikings which were clearly a different culture, arguably a different civilisation, but infused themselves in every corner of Europe, from England to Russia to Sicily and Byzantium.
Civilisations do not have clear boundaries, both geographically and temporally, and you certainly can't equate them with empires. I think civilisations rarely truly collapse (except under the impact of invasion or colonialism maybe), but they do change, and sometimes dramatically. The collapse of empires is sometimes dramatic, as in the case of the Western Roman Empire, and sometimes barely noticeable, as in the case of the Holy Roman Empire.
