>>510264588 (OP)>the king is the ruler, the richest and most powerful person in the countryWell, that's more of a modern idea. Throughout much of the Middle Ages, monarchs would struggle generation by generation, as revenue bearing land was split between numerous heirs of the previous king. While the monarch had vassals (the counts, barons, and others) under them, who under contractual law were required to field certain numbers of troops and certain amounts of taxation to the monarch, those vassals could become more wealthy and more powerful than the monarchs themselves in direct terms. This could cause significant instability, particularly if the monarch wasn't politically savvy, and his underlings became rebellious.
The system made itself vulnerable to money lenders, as monarchs dealt with both threats from neighbors, and from within their own realm. Christians lending money at interest to Christians was a no-no (and Jews and Muslims had the same rules), so they skirted the spirit of the law and took money from Jews, instead of examining the issue and solving it directly.