The key translates this as: "Not even any of the soldiers has stayed in the camp."
I am having some trouble. Is the genitive "militorum" used along with "castris" -- the soldiers' camp? The word order seems odd, but maybe there is no better alternative if you want to use the "ne... quidem" construction as this exercise does.
I think my problem is that I thought quisquam should correspond with "militorum," that is, it should be cuiusquam, but I see now it's a substantive and not an adjective. Is that right? So what the sentence is saying, to be literal instead of elegant, is "No one at all, not even its soldiers, stayed in the camp." You would not use a translation like this professionally, since it is very stilted, but I'm trying to see if I get all of the Latin's nuances, with the genitive and the indefinite pronoun quisquam somehow rendered into English.
I think in the key all those who do not remain in the camp are put under the umbrella of "not even..." where the quisquam becomes subsumed in the "ne...quidem." Am I on the right track here?