Complex sentence: On Jon's advice Stannis rallies the Mountain Clans and attacks Deepwood Motte, the last Ironborn-held refuge in the North, restoring it to the Glovers, thus winning their support and that of the Mormonts.
Split into simpler sentences: On Jon's advice Stannis rallies the Mountain Clans and attacks Deepwood Motte, the last Ironborn-held refuge in the North. He restores it to the Glovers, thus winning their support and that of the Mormonts.