How would you defend the North?
The North is so expansive that it seems to me an impossible place to defend from 100,000 inexhaustible zombies backed by a zombie dragon.
I would consider evacuating everyone much further south to a point where the land narrows and forces the Army of the Dead into a bottleneck. Maybe near The Twins. Send the noncombatants (elderly, small children) into the mountains of the Vale for protection.
https://atlasoficeandfireblog.files.wordpress.com/2016/02/westeros-org-map.jpg