- "A hundred years ago, another King-Beyond-the-Wall, Raymun Redbeard, realized the Wall's size is both it's greatest strength and it's greatest weakness. He waited for our patrols to pass and sent climbers to the now unguarded stretch of the Wall. When they reached the top, they dropped ropes and ladders for thousands more wildlings to clamber up. The Night's Watch didn't even know his army had crossed until after the Starks and Umbers had cornered and destroyed him, and that was a hundred years ago, when the Watch had many more men than now."
- ―Samwell Tarly
The Battle at Long Lake was a military engagement fought approximately 75 years before the events of Game of Thrones.
The numbers of the Night's Watch, which guarded the Wall, had drastically diminished since the Targaryen Conquest nearly two centuries before. The Watch used to have a steady stream of recruits from the losing sides in the endless wars between the independent Seven Kingdoms, but after the Targaryens united the kingdoms and put an end to the internal wars, the stream of recruits fell to only a trickle. As a result several of the 19 fortresses of the Watch used as bases for patrols had to be abandoned - even their original headquarters The Nightfort, the first castle along the Wall.
Raymun realized that the Watch's dwindling patrols were now spread very thin, with miles-long stretches of the Wall which were not patrolled at all for days. Therefore he sent climbers on the difficult ascent up the Wall - unnoticed and unimpeded by any fire from defenders above. The climbers then dropped ropes down which the rest of Raymun's wildling horde used to mount the Wall, then pass down to the southern side.
The Night's Watch didn't even notice that the entire wildling horde had crossed over the Wall. Raymun proceeded to lead his army south through the Gift and then into the North itself, whose lands and homesteads were taken completely by surprise.
Eventually the head of House Stark, as Warden of the North, led out his armies to stop the wildling advance. Along with the immediate forces raised from the Starks' own lands, they were joined by the forces of their vassals House Umber - famed warriors whose lands are located closest to the Wall, making them bitter enemies of the wildlings.
The Starks and Umbers attacked Raymun's hosts at the shores of Long Lake. The wildling army was destroyed and Raymun himself was killed.
Raymun Redbeard's assault was the last major attempt by a King-Beyond-the-Wall for almost a century, until Mance Rayder united the wildlings again as the new King-Beyond-the-Wall.
In the books
In the A Song of Ice and Fire novels, the Warden of the North at the time was Lord Willam Stark, who was joined by his vassal Lord Harmond Umber and his forces when he marched to head off Raymun Redbeard at the shores of Long Lake.
Raymun beheaded Lord Willam during the battle but was then himself killed by Willam's brother, Artos the Implacable. Raymun's sons were also killed during the battle, though his brother Red Raven survived and fled back north of the Wall (as Tormund explained, he only later came to be known as "Red Raven" because like his brother Raymun Redbeard he had red hair, and a bard who composed a popular song about him simply needed to use a word that rhymed with "craven", because he ran away). Given that the position of King-Beyond-the-Wall isn't hereditary it didn't really matter that Red Raven managed to get away, the other wildlings wouldn't unite behind him as they did his brother.
The battle was one of the worst hours of the Night's Watch, which hadn't even noticed the wildlings climb over the Wall and let them slip into the south unchallenged. It highlighted just how badly the Watch had dwindled by that point in time: what little honor and respect had still been held for the Watch up until that century largely melted away, and many openly began to scoff that it was a glorified penal colony, ineffective at performing its most central duty. The Lord Commander of the Night's Watch at the time was Jack Musgood, also held to be one of the worst Lord Commanders in the Watch's history for this debacle. He did lead soldiers of the Night's Watch south but they were far behind the trail of Raymun's army and only showed up at Long Lake after the battle was already long over. Artos Stark bitterly ordered Musgood and his men to help with the only remaining task: burying the dead, who might not even have died had they stopped the wildlings at the Wall or at least given some advanced warning that they were coming. Musgood was forever after nicknamed "Sleepy Jack", extending into later generations until the present day, to shame him for essentially falling asleep at his Watch.
Willam Stark's death in the battle was the latest in a string of tragedies to strike House Stark in the past quarter-century sine Daeron II's reign. Willam's own father Beron Stark was mortally wounded by ironborn raiders on the west coast, while Beron's own father Brandon only came to rule after his older brother Barthogan Stark was killed putting down a rebellion by the tribes on Skagos island. After the Battle of Long Lake, Willam's son Edwyle subsequently came to rule Winterfell, and Edwyle's son was Rickard Stark, father of Eddard Stark (making Willam the great-grandfather of Ned).
Leading up to the time of the Battle at Long Lake, as Willam's father Beron lay dying, there were already four different Stark widows each fighting each other over who should rule Winterfell as regent, with Beron's wife becoming the fifth: they came to be called "the She-Wolves of Winterfell". Upon Beron's death there were 10 different underaged Stark children who had potential claims to rule of the North.
The unlikely duo of Ser Duncan the Tall and Prince Aegon V Targaryen came to the North during this tumultuous time (in an as-yet unpublished prequel novella titled The She-Wolves of Winterfell). They may have met Willam Stark - and it is possible that Duncan and Aegon V directly took part in the Battle at Long Lake. If so the campaign against Raymun Redbeard and the Battle at Long Lake would appear in any potential live-action adaptation of the Tales of Dunk and Egg prequel novellas.
In Season 3's "The Bear and the Maiden Fair" - which was written by George R.R. Martin himself - Jon Snow tells Ygritte that six times in the past thousand years the wildlings have united behind a King-Beyond-the-Wall and attempted to force their way into the Seven Kingdoms to the south, and each time they have been repulsed. Ygritte doesn't have any specific knowledge of these past invasions, which is Jon's point: he says that every child in the North knows all of the great battles against wildling invasions, all of the names, places, and strategies - while the wildlings like Ygritte barely remember much detail about the campaigns, meaning that they will keep making the same strategic and tactical mistakes which the North has already learned to take advantage of when countering their invasions. Jon didn't mention the Battle of Long Lake by name but this is one of the more famous battles that repulsed a wildling invasion in the past thousand years.
Moreover, Martin's script in this episode introduced the new detail that there have been six Kings-Beyond-the-Wall who led invasions against the south in the past thousand years - previously the novels only mentioned Raymun Redbeard as the most recent King-Beyond-the-Wall before Mance Rayder. Jon may have been including the Horned Lord in this count - there is some debate among maesters whether the Horned Lord lived one thousand or two thousand years ago. Otherwise there are at least four, maybe five, previous invasions of the North which ware unaccounted for - but apparently none of them were as big as the campaign culminating in the Battle at Long Lake.