This one seem simple... When the Bolton army marched out to meet Stannis, they were defeated. I think Roose himself was captured by Stannis. That's why Ramsey wanted Jon to send Stannis family to him as prisoners. Ramsey want to do a prisoner exchange. What other reason would Ramsey want with Stannis wife and daughter if Stannis himself is dead? He can't randsom them because all the Baretheons is dead and Stannis is technically a trader...
And the banker from the Iron Island also found Stannis. GRRM would not have the Iron Bank travel around the world to find Stannis and he die a minute later... And have Davos go through all the trouble to find Rickon so Manderly can support Stannis...
Ramsey is surrounded, desparate, and starving in his castle full of people who would love to cut his throat... Things was bad in the castle when Roose were there... Imagine how bad it is with Roose held prisoner by Stannis...
And Stannis is too good a commander to be defeated so easy... It took the combined power of Highgarden and Casterly Rock to beat him at the Blackwater... And even then Tyrion is the one who saved the day... If it wasn't for Tyrion, Tywin would not have made it in time...
And the banker from the Iron Island also found Stannis. GRRM would not have the Iron Bank travel around the world to find Stannis and he die a minute later... And have Davos go through all the trouble to find Rickon so Manderly can support Stannis...
Ramsey is surrounded, desparate, and starving in his castle full of people who would love to cut his throat... Things was bad in the castle when Roose were there... Imagine how bad it is with Roose held prisoner by Stannis...
And Stannis is too good a commander to be defeated so easy... It took the combined power of Highgarden and Casterly Rock to beat him at the Blackwater... And even then Tyrion is the one who saved the day... If it wasn't for Tyrion, Tywin would not have made it in time...