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Checked the save file, and there are indeed no dragons (apparently, the computer acted first), and there's also no control magic. Some smart AI placed the Grail in a Castle. There's still a chance, but if the pink AI killed the dragons, it might also open the gates, but that's unlikely. I guess I'll have to replay it.
Generally, the standard Oceania map is played with control magic enabled. Someone must have edited the map.