In Game of Thrones season 3, episode 10, "Mhysa," Jon Snow makes it to Castle Black, badly injured by the Free Folk spearwife - Ygritte's arrows. He's covertly gathered a great deal of useful intelligence from the King-Beyond-the-Wall, Mance Rayder, and the Free Folk. Jon betrays Ygritte's trust by pretending to be a Night's Watch defector to get inside Mance's camp but chooses duty over love and leaves her to ride back to Castle Black. In the opening of Game of Thrones season 4, episode 1, "Two Swords," Ygritte grows impatient to march on Castle Black but Tormund Giantsbane says they must wait for Mance's signal. He knows she's spared Jon's life and let him go, and as they bicker, people of the Free Folk clan named - Thenns arrive at their camp.
Back at the castle, Jon responds intelligently when called upon to answer for killing the legendary Ranger, Qhorin Halfhand. Jon's friend, Maester Aemon, and enemies, Ser Alliser Thorne and Janos Slynt are the notable men at the tribunal. Jon admits to killing Halfhand, saying, "He wanted me to kill him... The Halfhand believed our only chance to stop Mance was to get a man inside his army," adding:
While we sit here debating which rules I broke, Mance Rayder marches on the Wall with an army of 100,000.
Jon (Kit Harington in Game of Thrones) tells his brothers that the Wildlings are more disciplined, and Mance has organized the rival clans such as the Thenns, the Hornfoots, the Ice-river clans, and the Giants into an army and that an attack on Castle Black is imminent.
Jon's Inputs
Jon tells Maester Aemon, Alliser Thorne, and Janos Slynt that a band of Wildlings is stationed south of the Wall, led by a man named Tormund Giantsbane. He says he killed their warg and a few more people before escaping to Castle Black.
Their orders are to attack Castle Black from the south when Mance hits it from the north. The signal for the attack will be a bonfire. Mance said it would be the greatest fire the North has ever seen. That's the truth. All the truth.
Maester Aemon (Aemon Targaryen) remarks Jon's offenses including breaking the vow of celibacy are trivial as he was acting on Halfhand's commands. The Acting Lord Commander, Alliser is disappointed as he wants to execute Jon.
Jon trains men at Castle Black, as his time with the Wildlings has given him a firm grounding in their way of combat. He tells the brothers how a lot of them fight with a weapon in each hand, while Alliser takes every opportunity to disrupt his work. He dispatches Jon beyond the Wall to the Craster's Keep, saying he wants him to get to the Night's Watch mutineers before Mance. As the mutineers have critical information on the Watch, Jon fears if Mance learns about their defenses, he will come at them with all he has. It is pertinent to mention that Alliser sends Jon to Craster's hoping he dies in the mission, thus paving the way for him to be elected as the new Lord Commander of the Night's Watch. However, Jon and the volunteers succeed in Game of Thrones season 4, episode 5 "First of His Name" and return to Castle Black in Game of Thrones season 4, episode 7 "Mockingbird."
Jon informs the brothers that Mance's army was near Craster's Keep when they left. While he says they must be on the defensive, Alliser is in denial. Jon and his friends suggest they should seal the tunnel, but Alliser has the proposition vetoed. While Alliser and his men refuse to pay heed, Tormund, Ygritte, and the Thenns raid the nearby Mole's Town brothel in Game of Thrones season 4, episode 8 "The Mountain and the Viper."
The Castle That Stood For Thousands Of Years
The Night's Watch does not have the numbers, and Jon, Pyp, Grenn, Eddison Tollett, and Samwell Tarly know they do not stand a chance. Sam and Jon watch atop the Wall, and Ygritte tells Styr and his men that her former lover is hers to kill. Gilly makes her way to Castle Black and just as she reunites with Sam, two horn blasts alert the men of the Watch. True to his word, Mance lights the greatest fire and his army bears down on the Wall. Alliser admits to Jon that he should have let him seal the tunnel while they had the chance. Nearly halfway through Game of Thrones season 4, episode 9 "The Watchers on the Wall," Ygritte goes snooping about and tells her party that Castle Black's walls are penetrable and only some men are in the courtyard while the rest are up top.
Amid the great fire, the Wildling army, consisting of giants riding mammoths emerges through the woods. Alliser orders his archers to knock and hold, as Janos informs him that the Free Folk are attacking the southern gates. Ygritte kills many Crows with flaming arrows and Alliser raises the morale:
Brothers! A hundred generations have defended this castle! She's never fallen before! She will not fall tonight! Those are Thenns at our walls. They eat the flesh of the men they kill. Do you want to fill the belly of a Thenn tonight?
Alliser rallies men behind him, and the Wildlings breach the gate. Jon meanwhile commands the archers up top to stop Mance's army from attacking the gates and while a few attackers die, the rest attempt to scale the Wall. The archers are let down perpendicular to the Wall and a giant's arrow kills a man of Watch. Pyp dies in Sam's arms and giants and a mammoth begin pulling at the gate with ropes. Alliser (Owen Teale in Game of Thrones) and Tormund engage in a fight, and it's a bleak situation for the Watch.
Jon Fights In The Battle
In the final phase of the battle, Sam rushes to tell Jon that Alliser has fallen and that the castle won't stand much longer. Jon rushes down to fight the Wildlings and entrusts the responsibility of the Wall to Edd. Finally, Jon draws his sword, and orders Sam to release Ghost. After killing Styr, he stops as he sees Ygritte (Rose Leslie in Game of Thrones) has her arrow pointed at his heart. She struggles to shoot but is hit from behind by Olly's arrow. Jon rushes to cradle her, and she reminds him of their time together:
Do you remember that cave? We should have stayed in that cave.
"We'll go back there," says Jon to which she replies, "You know nothing, Jon Snow." Edd uses a giant scythe to slice the Wildling climbers off the Wall, and Mance's troops retreat. Jon has Tormund put in chains and takes stock in the morning. He knows Mance was merely testing their defenses and sets out to seek him. He hands the Longclaw to Sam (John Bradley in Game of Thrones) and walks out of Castle Black in the Game of Thrones season 4 finale an unhappy man.