Harrenhal

Harrenhal is a huge castle, the largest castle in all of Westeros, though it is also the most ill-omened. It is located on the northern shore of the lake known as Gods Eye at the heart of the Riverlands, south of the River Trident and north-west of King's Landing.

Harrenhal is believed to be cursed. It was built centuries earlier by King Harren the Black. He was betrayed by his bannermen during the War of Conquest and the castle was largely melted by dragonfire: while Harren had thought the castle's walls were tall enough to withstand any assault, he neglected to consider that the Targaryens' dragons could fly right over them. As a result Harrenhal is half-ruined, with stone towers melted and twisted by the flames of the dragons, giving it an ominous and nightmarish appearance. No lord has held the castle for long since Harren's death.

It was held by House Whent until the outbreak of the conflict between House Lannister and House Stark, when its title was given to House Slynt as a reward for betraying Eddard Stark. However, due to the area being a warzone, Slynt never actually obtained the castle. As of Season 2, it is being used as the main forward base for Tywin Lannister's army in the Riverlands during his campaign against Robb Stark.

Season 1
Catelyn Stark recognises a knight of Harrenhal at the Crossroads Inn and asks him if he is loyal to Lady Whent and Catelyn's father, Whent's liege lord. When he replies this is the case, she asks his aid in restraining Tyrion Lannister and receives it.

King Joffrey Baratheon gives the castle of Harrenhal to Janos Slynt, commander of the City Watch, for his part in betraying Eddard Stark.

Following the Lannister defeat at the Battle of the Whispering Wood, Lord Tywin Lannister announces that he will take his army of 30,000 men, capture Harrenhal and use it the forward base for his army in the Riverlands while Robb Stark's numerically superior forces consolidate near Riverrun.

Season 2
Arya Stark, Gendry, and Hot Pie are taken prisoner and taken to Harrenhal.

Behind the scenes
Production Designer Gemma Jackson said that in order to develop the look of Harrenhal, she visited Cambodia and the ruins of Angkor Wat to get a feel for how a vast but crumbling castle complex should appear, with many broken walls overgrown with vines and creepers.

In the books
In the Song of Ice and Fire novels, Harrenhal is a colossal castle consisting of five massive towers (the Tower of Dread, Widow's Tower, Wailing Tower, Tower of Ghosts and Kingspyre Tower) and immense walls.

The castle was built over 300 years ago by House Hoare, the ruling house of the Iron Islands which had overrun and conquered the Riverlands. It took three generations to construct. King Harren the Black completed the castle, boasting that it was impregnable. Unfortunately, the day he completed the castle was the same day that Aegon the Conqueror invaded Westeros. His dragons were not obstructed by the towering walls and roasted Harren alive in the tallest tower of the castle, now known as the Kingspyre.

Since Harren's death the castle has passed through a number of other noble houses (Qoherys, Towers, Harroway, Strong and Lothston), each of which has come to ruin. prior to the War of the Five Kings House Whent, loyal vassals to Lord Hoster Tully of Riverrun.

Harrenhal is seen as something of a white elephant: while it is technically the biggest castle in Westeros, it is so ridiculously large that the surrounding area cannot possibly produce the necessary food to support the massive army needed to fully man it. Rulers of Harrenhal usually lose money and resources simply by possessing it. As a result, Harrenhal is rarely if ever fully manned.