To avoid a war in terminology, lets say that we can consider a pre-feudal / feudal / post feudal some kind of monarchy, for the sake of learning.

So, german kingdoms which lasted (franks, visigoths, etc.) developed a structure based in these aspects, which are related:

• romanized but new structure: even though these new kingdoms were more or less "romanized" (thats a term we can argue about), they lost the importance of the city. People left cities (which were so important in the Roman Empire) and went to the "countryside". There, they wont have strong public relations, as they had in the Roman Empire: political relation is now private, and so is religion, and so is economy. And that private relation is a new type of relation, that will become "feudo-vassal": a relation of (more or less) dependance from a powerful landowner.

• primitive feudal relations: these relations of dependance created a figure: the so-called Lord, that will actually control his land and those who live in there. That is the origin of feudal relations, and vassalage will come up from there. But, to be honest, vassals are a harsh topic in historiography, and I wouldn't like to say things here that I dont approve myself. So lets consider that these primitive relations of power are the origin of the medieval lords. These relations are strong and ritual.

• soldiers as a actual "caste": in the dawn of monarchies, it the "soldier class" (let me be simple) will actually be the one leading the way. Being armed and fighting in wars grants the soldier the right to participate in politics. This caste is so important: being a soldier means being powerful and having property, and the future evidence for the importance of this caste will be the chivalry. Chivalry borns just as "being owner of a horse", which even in the Roman empire was a sign of power. Chivalry and knighthood will obviously develop.

Relations of power among chivalry, the need to organize the territory and this so-called feudalism (the Lord above the people) will be the origin of the monarchy. Monarchy, as we understand it in medieval ages. These "knights" will be the new "nobilitas" conception: being able to participate in politics is reserved to those who fight for our "nation" (please, do not understand nation as its modern concept). Between the "knights", there is one "legitimately" called King, but this is not a emperor: Kings are controlled by its court (of knights who are trying to take over the power) and relations between the royal court are the materialization of politics in the Middle Ages. (obviously, there is much more).

Ok. Middle Ages is my specialty, but English is truly a barrier for me. I expect I've made myself understandable. Please, ask anything :D My sources are, as I love them, Marc Bloch's Feudal Society (gold) and some other authors around him