Where “feudalism” refers to a specific form of society and not just “that time period”
I should note that Marx’s writing in The German Ideology forms the basis of what would later come to be known as historical materialism, but Capital expands on it a lot, so if you’ve already read Capital you might find The German Ideology to just be a repeat of the same (and it’s not nearly as detailed in its analysis). I generally recommend it before reading Capital as I personally found it a lot easier to understand the concepts beforehand, rather than trying to tackle Capital first.