Riot of King's Landing

The Fall of Winterfell was an event during the War of the Five Kings. To gain the respect of his fellow ironborn and his father, Theon Greyjoy. It was not so much a battle as a surprise seizure of Winterfell when virtually all of its guards had been lured away due to a feint at Torrhen's Square.

Background
King Robb Stark sent Theon Greyjoy to Pyke with the offer of an alliance to his father, Lord Balon Greyjoy of Pyke. Lord Balon however, had plans of his own and rejected the offer.

Theon joined his father but was given a single ship, the Sea Bitch, and ordered to raid the Stony Shore. Meanwhile, Theon's sister Yara was given command of thirty ships and tasked with taking Deepwood Motte, the seat of House Glover.

Following the suggestion of his first mate, Dagmer Cleftjaw, Theon abandoned the idea of raiding the Stony Shore and devised a plan that would win him the respect of his crew: feign an attack on Torrhen's Square, the seat of House Tallhart, to draw away the Stark men defending Winterfell.

Winterfell falls to the Kraken
Just as Theon planned, Winterfell sent a force of two hundred men led by Ser Rodrik Cassel to drive away the unidentified attackers of Torrhen's Square. With the castle undefended, Theon seized the chance and sent his men with grappling hooks to climb over Winterfell's walls and let Theon inside.

As Theon's crew rounded up the members of the Stark household, Maester Luwin managed to send a raven with news of the attack and the identity of the invaders. Theon, styling himself ' Prince Theon' convinced Bran Stark to yield the castle to spare the lives of his subjects.

In the books
In the books, the Fall of Winterfell takes place in a similar matter. The ironborn do not face Summer and Shaggydog, Bran and Rickon's direwolves, because they had been locked inside the godswood after Shaggydog had attacked one of the Frey wards of Catelyn Stark.

Also, Ser Rodrik is not executed after the taking of the castle. Following the events of Torrhen's Square, Ser Rodrik begins gathering men to retake Winterfell.