Sub-Roman Britain (Cavalry etc)

[cagwinn]

Castrum, to me, in Latin useage, is a camp or garrison.

But your version of Ker might work. Its an old aristocratic family name in UK. It could have origins in Christianity..Ker= priest in Indo aryan and some Turkic dialects. Also has to do with word Black in slawic languages which, in old proto versions, can also use it as Priest. The black word is interesting because it crops up as in blacksmith and various other metalworking "magic" vocab.

I never did work it out. metallurgy, magic and religion wre often mixed, which complicates thing a bit.

Oh, the surname Ker®...my understanding is that it's either Gaelic, perhaps from Irish ciar "dark, brown", or Norse (Old Norse kjarr "marsh [dweller]").