In Game of Thrones season 6 Ramsay was about to destroy Jon Snow and his whole army, when the Knights of the Vale suddenly appeared, and reversed the tide.
The key point was the element of surprise: if Ramsay would have known a big enemy army arrives, he would have stayed inside Winterfell and his downfall would have been prevented.
The Knights of the Vale are not exactly commando; They have many horses, they make lots of noise; They can't really hide their movement.
So how come it was a surprise attack?