I'm taking a look at the unsatisfying NaNoWriMo fantasy novel I wrote last year. As part of that I have a few things to think about, some old, some new, and I'm doing some of it in public, here. So last December amongst other things I had this to say about feudalism: "In a feudal setting the greatest crime is to betray your lord."
Let's be clear on this; chivalry and honour are the bright cover on power coming from a naked sword. Nevertheless, that cover is important. One does not simply betray an oath. One works around it, obeys the word, not the spirit, blames it on someone else, claims one was under duress and gets the Pope to absolve you of it. You try your best to avoid giving it, negotiate the wording, bribe someone to interfere with the proceedings. You pretend ignorance, make excuses about the weather and the harvest, say you would have fulfilled your obligations, but... You don't flat out break the oath. Oathbreakers are accursed and damned. If you don't keep your oath, even in the breach, why would anyone take oath with you?
But what feudalism is really about is an exchange of land for service. The king grants land to his lords, who pay his taxes and call up armies when he needs them. The oath is just the vehicle for this exchange. All the rest of it - powers of justice, decentralised authority, monopolies, rights to tolls, fish, hunt, graze etc. is about what you can do to run your land. So importantly, when we have the feudal system and oaths, we have chivalry, but we also have lawyers.
 This is more complex than it sounds. There are of course, regular taxes, tolls, dues etc. but to pay for wars and so forth, the King will often have to declare a tax at the very same time that his vassals are taking the most productive workers out of the fields to form an army. Not surprisingly this is unpopular at the best of times, and during the feudal period it was never the best of times. Worse than that; taxing income was not the usual way things were done, what with it mostly not being a cash economy, and dependent on the harvest(s). Instead a proportion of your wealth is what was taxed. Peasants, of course, don't own anything, so they don't have to pay a lot. Lords, who actually own stuff, do. However, you don't get to be a lord with lots of stuff by giving it away, so you get it back by squeezing the peasants, which is why they don't have much stuff in the first place.