"Essentially a motte-and-bailey"

A M&B is a medieval castle layout. Those bloody Norsemen immigrants who duffed up those bloody Saxon immigrants, wot duffed up the native Britons, built quite a few of those things. Something, something, Frisians, Romans and other foreigners. Everyone is a foreigner or immigrant in Britain apart from us locals, who have been here since the big bang.

Anyway, please explain the analogy.

(https://en.wikipedia.org/wiki/Motte-and-bailey_castle)

TIL. Thanks for sharing.

https://en.wikipedia.org/wiki/Motte-and-bailey_fallacy