"(...)for a millenium it was the most developed part of Europe (...)" "For reasons that are not entirely clear it remained a medieval, stagnating center" I thi

"(...)for a millenium it was the most developed part of Europe (...)" "For reasons that are not entirely clear it remained a medieval, stagnating center" I think you made it quite clear. It was very developed very early, and it got complacent. When competition came knocking it had no further source of advantage and quickly lost ground until it was gobbled up by a rising polity.