This is a very important point. Most people think of all society from tribes up to 1600CE as one big "it was basically all kings"/medieval/dark ages bloc. Very few people could actually tell you about the development through those various eras, such as the philosophies, societal movements and technologies developed, invented, and at play throughout those millennia (including myself, I know shit all).

To ask "what was feudalism in decay" is like asking "why is glass transparent"; It sounds simple, but you need to understand a whole bunch of other shit first for the answer to actually be meaningful.