I would use a hyphen myself: "...most keenly fought-over battlegrounds", and maybe that shows the answer right there. If not, the explanation is that it is a military metaphor, and here it means that "individualism" has a number of tenets that it has to defend against its opponents (with a more collectivist or societal mindset, perhaps?), and the missing subject of this sentence is one of the more contentious ones, and is important to the protagonist.