Where to find the 'keep surrounded by mountains' in Total War: Warhammer 3 Thrones of Decay

Playing as Elspeth Von Draken in Total War: Warhammer 3's new Thrones of Decay expansion, you'll find yourself scouring the Empire in search of a keep surrounded by mountains. After you've completed the first few quests in the Graveyard Rose's campaign, she'll be confronted with a vision of a mysterious crown, but the quest won't tell you exactly where to find it.

Your only clue is that there's a mountainous keep in Elspeth's vision. So, you'll have to track it down, send an army there,...