When the Carolingian state collapsed and the Roman tax system finally died with it, medieval Europe faced an awkward problem: how do you defend a settled population against Viking raids when you cannot pay an army? The solution, worked out across the ninth and tenth centuries, was feudalism — a graduated system of land grants, in which a king granted estates to nobles, who granted smaller estates to knights, who in turn provided military service when the king called. Land replaced money as the primary instrument of state finance.
Underneath the noble pyramid was manorialism — the actual economic engine. A manor was a self-contained estate where a lord owned the land and peasants worked it in exchange for protection, the right to farm strips for their own families, and a share of common woodland. The peasants were tied to the land — they could not leave without permission, and were sold along with the estate when it changed hands. This was serfdom, the dominant labour relation in much of Europe for nearly a thousand years. The system was not centrally designed; it emerged from a thousand local bargains, codified into custom, eventually into law. It produced a remarkably decentralized political order in which kings were often weaker than their major dukes, and dukes negotiated with their vassals as near-equals. The medieval European state was not the absolutist monolith later monarchies pretended to inherit; it was a negotiated patchwork.
The European tradition of constitutional limits on royal power — Magna Carta, the parliaments, the courts of the realm — emerged from feudalism's structurally negotiated character. Kings who needed their nobles' military service had to bargain for it, and the bargains became precedents, and the precedents became rights. Modern constitutional democracy has unlikely roots in the most exploitative agricultural arrangement in European history.