I captured 5 castles. Indeed, before this, they were positioned in such a way that I had to use a hired hero to weaken either the blue or the red faction, and the main hero would finish them off when there was no way to connect.
And by the way, why is there a blue hero on one of the islands (I've only explored one island so far)?
How many Archdevils will there be?
Hmm... It turns out it's all very simple.
At least they gave me a wave of Arcane units after the last battle.