My friends and I are disagreeing on how to handle a Stonehedge with the Royal power. Relevant rules:
"Popular Places' powers cannot be used by Races that are in Decline".
Royal reads "If you go in Decline, your Queen stays where it was and keeps that Region immune, but can no longer be moved".
Stonehedge reads "if the Special Power associated with Stonehedge is a Power that has an effect on In Decline Troops ..., the player only gets to use the Stonehedge's power once his Troops are In Decline."
Suppose I control the Stonehedge, and put the queen on the Stonehedge region. Then, I go in Decline. Does the Queen stay, and make the region immune? Or, does the Queen get removed up, since I'm not allowed to control the Stonehedge with a Declined race?
They get to stay. I'm guessing the issue your friend has is that the rules cover abilities with no in-decline attributes and abilities with only in-decline attributes but not ones with both.
The line about in decline races not being able to use places is overruled if a place explicitly states it can be used by in decline races and in the case of Stonehenge it's pretty clear that it was intended for units to keep in decline bonuses while their unit was in decline.