The Second Battle of Deepwood Motte is a battle during the War of the Five Kings. It is mentioned in the episode "Home", and sees the liberation of the Northern castle Deepwood Motte from the Ironborn, ending King Balon Greyjoy's invasion of the North.
In response to Robb Stark's campaign to overthrow the Lannisters and establish Northern independence, Balon Greyjoy takes the opportunity to attack the poorly-defended North, and has his daughter, Yara Greyjoy, take thirty ships to attack and take Deepwood Motte. Though the castle is taken, Yara returns to the Iron Islands following the Sack of Winterfell and House Bolton's retaliation against the ironborn, leaving a garrison to hold the castle.
House Glover, with help from the Boltons, eventually attacks and retakes Deepwood Motte from the ironborn, killing them all. Balon and Yara are informed via a raven.
As Deepwood Motte was the last Northern castle held by the ironborn, this effectively marks the end of Balon's occupation of the North, all of his men having been driven out by either House Bolton or the local northern Houses. Though Balon vows to strike back, he is killed shortly afterwards by his own younger brother, Euron Greyjoy.
In the books
In the A Song of Ice and Fire novels, Deepwood Motte is liberated by Stannis Baratheon's army on the advice of Jon Snow, reinforced by the Northern mountain clans. Asha and her men fight valiantly, but being outnumbered and surrounded by unfamiliar territory and darkness, they are defeated. Nearly all Asha's troops and ships are destroyed, and she and her surviving men (less than ten) are taken prisoners. The ironborn garrison at Deepwood Motte numbered about 200 men before the battle.
During the battle, Stannis receives unexpected (but welcome) assistance from Alysane Mormont, Maege's second daughter. He writes Jon about that: "Alysane Mormont, whose men name her the She-Bear, hid fighters inside a gaggle of fishing sloops and took the ironmen unawares where they lay off the strand." Apparently, despite what Lyanna wrote Stannis, the Mormonts have changed their mind about supporting Stannis.
Unlike mentioned in the show, the Glovers do not take part in the battle, and neither are the Boltons. Perphas Ramsay intended to liberate Deepwood Motte the way he did at Moat Cailin (as written in a letter Asha receives from him), but Stannis beat him to it.
The battle takes place long after Balon's death, while in the show it happens before.
Deepwood Motte is restored to House Glover, but Sybelle's children Gawen and Erena are still held by the ironborn at Ten Towers, brought there by Asha prior to the battle.
The victory grants Stannis credibility in the eyes of the people of the North, and many survivors of the northern houses join him in his campaign against the Boltons, increasing his host to about 5,000 soldiers, more than three times it originally was.
Asha believes that her defeat has destroyed any hope for her to rule the Iron Islands. The ironborn are not a forgiving people, and she has been defeated twice: once at the kingsmoot by her uncle Euron, and again at Deepwood Motte by Stannis - more than enough to stamp her as unfit to rule in the eyes of the ironborn.
There is one more stronghold still held by the ironborn in the North: Torrhen's Square, which was mentioned as being attacked by the ironborn in Season 2, and in the books was taken by Dagmer. As of the most recent novel, Dagmer still holds Torrhen's Square. Asha offers Stannis to have Dagmer surrender, but Stannis refuses, having no interest in Torrhen's Square. The TV series apparently just condensed this so Torrhen's Square was liberated off-screen, and officially the ironborn have no more presence in the North.
In the novels, the liberation of Deepwood Motte is the one time so far that House Forrester has been mentioned in the main narrative - a minor House who serve as vassals to House Glover, who are the main characters in the tie-in video game Game of Thrones: A Telltale Games Series. After Deepwood Motte is liberated, Lady Sybelle Glover provides Stannis with guides and "hunters sworn to Deepwood with clan names like Forrester and Woods, Branch and Bole." A future "season" of the Telltale video game might tie-in with these off-screen events - though the first "season" of the game was set during Season 4 of the TV series, so these events around Deepwood Motte and the Wolfswood would correspond to a potential third season of the video game.