Libertatem hierarchia. What we call European feudalism was the becoming–patchwork of the Roman Empire. As the control structures of the Roman Empire atrophied, and the de facto liberty of its subjects increased, those subjects sorted themselves according to their objective levels of political power. Slaves sorted into Serfdom, mid-tier Roman nobility sorted into Lordship, and Roman super nobility sorted into Overlordship, and so on. Feudalism was obviously hierarchical, but also free in the sense that each member of each level recognized themselves as rightful members of each level. Hilaire Belloc paints a succinct but colorful portrait of this hierarchy from below, summarizing European feudalism thusly:
the passing of actual government from the hands of the old Roman provincial centres of administration into the hands of each small local society and its lord. On such a basis there was a reconstruction of society from below: these local lords associating themselves under greater men, and these again holding together in great national groups under a national overlord.
In the violence of the struggle through which Christendom passed, town and village, valley and castle, had often to defend itself alone.
For the purposes of cohesion that family which possessed most estates in a district tended to become the leader of it. Whole provinces were thus formed and grouped, and the vaguer sentiments of a larger unity expressed themselves by the choice of some one family, one of the most powerful in every county, who would be the overlord of all the other lords, great and small.Hilaire Belloc, Europe and the Faith
One might raise many good questions about Belloc's historical narratives, which I admit are relatively light on primary sources and heavy on romantic nostalgia. But with respect to the idea that European feudalism was a free hierarchy, this must have been the case because if any member of any level possessed greater power than their assigned status reflected, they could have just taken their rightful status: that is what the decline of Roman authority implied. If a Serf thought they were incorrectly or unfairly lumped in with the Serfs, they were perfectly free to create and secure their own manor against raiding pirates and crusading Islamic armies. Of course, having recently been slaves, they had no such power. Having no such power themselves, and having no centralized authority to physically protect their survival, it seems fair to presume they genuinely wanted to ally with someone who could raise an army.