Based on my experience with trying to buy hunting land, setting up a trust and leaving your property to the trust is a great way to stop your kids from "selling the family farm" after you kick it because there seems to always be one guy in every trust that wants a bigger percentage of the sell than everyone else which quickly turns into a real life example of the prisoners dilemma.
Castle estates are probably a few generations deeper than US land trusts which would make the problem much worse to deal with.
It sounded to me like they could live there if they were family of the original inhabitants but not sell that right to outsiders?
Like it's held in some kind of trust for them.
Though it's not spelled out exactly in the article either way.