>>520486103
Looking around the data a bit more,
I'm also fairly sure they just filtered with "historic=castle" and not "building=castle"
That poses a problem, because you'd ideally want things like sites, buildings or ruins.
Which means, the is basically just fucking anything vaguely related to castles. It might just be a map of old moats. It lines up very well with the data, so I'm fairly certain I'm right on the money there.

Just to emphasize why this is a problem, it didn't take long to find this little shitty islet at 44°44'33.3"N 0°41'36.4"E
This thing is a fucking castle according to that logic.