It was Unaroyal's turn during the Parley. Note how Rob carefully said that the Unaroyal forces leapt across the hex boundary to re-croak the princess.
Then the Queen saw what was going to happen, promoted her troops, sent them outside, sent her casters off, wrote her letter, and killed herself.

Wanda wasn't being James-Bond-Villain-level stupid; she was hoping to take the city without a fight (because she can't de-crypt her troops again). She didn't think of the possibility that the queen would be so upset at seeing her daughter de-crypted that she'd react that way... but that's consistent with her character.

And who knows, maybe this is that one thing Parson advised against and they did anyway...