According to this, as soon as Jon and Daenerys learned that the wall fell down by the Eastwatch, they immediately arranged old ones and the children of Northeners to evacuate Westeros. Many ravens were sent all around the continent and although most of the North was already lost (and also Winterfell) , a good amount of people still menaged to reach the White Harbor.
Then, while people were about to board on ships, all the hell broke loose. Our remaining heroes were ready to die on their last stand but there was someone else in White Harbor, too -who came back from Volantis for help in this dread hour. Melisandre and red priestesses of Rhollor marched Fiery Hand on battle. She casted down her new ability on Fiery Hand soldiers without a second thought, she even forced herself too much to revive dead bodies.
And she made the ultimate sacrifice while the attempt.
At least, in my ending. :)
Hope you guys like it.