Perhaps it was all Samwell's dream and the battle hasn't started yet.

Maybe because Arya killed the NK she now 'owns' him and the dead (who all come back to life) and March on Cersi together!

(She appeared out of nowhere past all the NK's bodyguards)

Loads of let downs and 'false plots' for me in this episode.
They could have been much cuter with the death of the NK. Arya posing as his henchman with a white walker's face on?