What are some plausible PODs for extending the political, economic, cultural, philosophical, religious, military, etc. patterns of the European Middle Ages - in particular the High Middle Ages - as long as possible into OTL's Modern-era centuries? That is, what is the best way to keep Europe (and surrounding lands) in the Middle Ages past when they ended in OTL? Any POD from AD 500 to 1500 is welcome, from any part of the world.