Yeah, I agree with luzula about 'spy'—the problem seems to be that Ewen is being interfering and over-solicitous about the Prince's safety, not that his loyalty is in question. (His 'something that no gentleman can possibly call another' makes me wonder if it's a word that was in established use as an insult, as it were—in which case it could be some obscure term common amongst eighteenth-century gentlemen but not much in use now! Who knows...).

Anyway—Ewen thinks that it would take a certain amount of skill and courage to lead a raid from the Castle, and behold!

Haha, good point :D But poor Keith must have been pretty disappointed at thinking he'd got Charles himself (I can imagine the message reaching him in a way that implied more certainty, e.g. perhaps the supposed Prince had confirmed his own identity) and then finding out he hadn't. He is certainly a bit rude there!