Stannis has Mance Ryder (They burnt a fake in the book) alive and infiltrated the garrison. Stannis has already used subtlety to defeat the Ironborn Garrison at Deepword Motte and based on the Pink Letter, theory I think Stannis will recover the Frey armour etc and have his own men plus the Manderlys infiltrate Winterfell too to cause chaos, sabotage defence, open the gates etc, this is purely my head canon though.