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Foundation is correct.Here is the complete and comprehensive list of restrictions on secession:You must be Duke.Your Duchy must contain at least one city.Your Duchy cannot contain all the cities in the realm.Your Duchy cannot contain the realm's capital.You must be in the city (within your Duchy) that you wish to become the new capital of your realm.This city must not have high independence/low control.You must have been in both your duchy and realm for at least 2 weeks.You must have at least 6 hours available.You cannot already be the ruler of your realm.You cannot be in an NPC realm (pretty much moot nowadays, save for bugs).You cannot have a family member who is already a ruler on the continent.
So strongholds cannot secede to form a new realm anymore?
hits both sides, doesn't it?