Heroes 5.5 WarGame Edition - Tactical Edition.

It's my first time playing 5.5, and I have a question:

How does the AI convert captured castles to its faction? A demon hero runs into a Dungeon, and on the next turn, I already see a demon Dungeon.

I've looked everywhere, and I don't see any buttons or prompts to change the castle's faction, but the AI converts everything it captures.