Terraniux
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Why? Who would give their city to an ally?;)
I would, on the contrary, add the option to take artifacts from an ally during a trade, because in the last game, his heroes had 2, or even 3, Spheres of Vulnerability each, and I had none! :( And since it's AI, asking him to share is useless!
I once lost like that. At night, the enemy captured my city, and then it went to an ally. And if I had recaptured the enemy city, I wouldn't have been able to recapture the allied one. And finding/capturing new cities is unrealistic on Hard difficulty.