a) You can't do that, because the game requires at least one city or stronghold to remain with the realm.
b) Do you not get the difference between stealing the main duchy out from underneath its outraged ruler and deliberately planning the total merger of two realms?
a) good to know
b) if such was indeed the case. I can't say one way or another, I wasn't there. In any case, I'm happy for reduced tax efficiency and a new capital farther from the front lines. I'm sure that'll make the Lurias so much better against us.
