I always wanted to do a true diplomats, spies, and assassins mod where they'd be invisible except to other DSA units. But apparently the invisible units thing is also wrapped in enigmaticness and dll hard-coding.
I hated the espionage system is CIV3, thought the one in CIV4 was anemic at best, and was really disappointed with the implimentation in CIV5 G&K. I just hate the 'theoretical' treatment of espionage. Ought to be a game-board mechanic in my opinion.
err....end of rant.
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Anyway, Leugi, even if you made them look like Barbrians by using a Barbarians art_define, would not the team color still show on the unit flag? And wouldn't the other players still "know" who the unit belonged to?
As I recall I think you have to declare war before you can pillage a trade route. So even if you declared war, pillaged, then restored peace all as part of a new unit-panel action (and direct lua command for the AI), wouldn't that game-breakingly spoil diplo relations for the rest of the game between those two players because of the "we were at war at least once" diplo-hit?