"In Honor of Our Fathers" - "Wrath of the Heavens" [Campaign 3, Mission 3]

I have a problem: I destroyed the red castle, and the map is fully explored (I don't know about the dungeons), but I can't capture the blue castle in the center. I click on it, but nothing happens. Is this a bug?

Added after 42 seconds
I also conquered all the surrounding areas and killed the heroes.