Ethiopian troops?

[D B Campbell]

Could it be that the translation has given Luguvallium because Carlisle is close to Burgh-by-Sands, which had a numerus of Moors to supply the 'Ethiopian'? Sort of a circular logic there perhaps.

It seems to be an unnecessary emendation. I don't have the apparatus criticus in front of me, but changing apud vallum to "apud Luguvallium", seems to be a step too far. You are right -- the reasoning is definitely circular.

But if vallum itself denotes the wall, why also murum?

Using precisely your logic (i.e. why mention a murus and a vallum, when they denote the same thing?), the murum of the original has convincingly been emended to Maurum, to give us our Moor.

I am inclined to agree, that the "Ethiopian" has been added to emphasize the fact that we're talking about a black man. This detail is required for the point of the story -- i.e., that Severus' death is being foretold in front of him.