The largest castle in Westeros - Harrenhal is in ruins and comes into the spotlight during Tywin Lannister's war in the Riverlands. At the Battle of Whispering Wood in Game of Thrones season 1, Robb Stark captures Jaime Lannister, and in retaliation, Tywin decides to regroup at Harrenhal. As Robb's forces consolidate near Riverrun, Tywin decides to use Harrenhal as the base for his army. The castle is charred as it was burned down during the Conquest by Aegon Targaryen atop his mount, Balerion the Black Dread.

The construction of Harrenhal was commissioned along the northern shore of the Gods Eye Lake in the heart of Riverlands at the whim of Harren Hoare's grandfather. It was completed during the reign of Harren who went down as the last King of the Isles and the Rivers. Harrenhal was built on the backs of Rivermen, without any gains to them. In House of the Dragon, Lord Larys Strong noted that blood was mixed into the mortar of Harrenhal and that it is said to be a cursed place. While Harrenhal was built with the objective of solidifying House Hoare's grip on the Riverlands, Aegon, Visenya, and Rhaenys' arrival in Westeros changed everything.

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Aegon's Conquest

Harrenhal's construction took forty years and on the day its last brick was laid, Aegon and his sister-wives landed on the mouth of the Blackwater Rush in Westeros. After defeating nearby Houses, Aegon set out to conquer the Kingdoms. While marching to Harrenhal, Aegon was attacked not once but twice by House Hoare on the south shore of the Gods Eye Lake. Though Harren called upon the river lords to defend Harrenhal, they refused the call and rallied behind Lord Edmyn Tully of Riverrun, and together they supported the Targaryens in the Conquest (reported to be an upcoming Game of Thrones prequel). With this, Harren hid behind his impenetrable fortress, thinking he was prepared for a siege. Aegon arrived at the castle's gates and demanded his surrender:

Yield now, and you may remain as Lord of the Iron Islands. Yield now, and your sons will live to rule after you. I have eight thousand men outside your walls.

The obdurate Harren defied Aegon's command saying, "What is outside my walls is of no concern to me. Those walls are strong and thick." While Aegon warned him Harrenhal's walls wouldn't be able to stop dragons, Harren said dragonflame couldn't burn stone.

Aegon promised to end Harren's line and true to his word, he flew over the castle at sunset, poured Balerion's fire on it, and melted its towers. Harren and his sons who hid inside the largest of the towers of Harrenhal were charred. The burning of Harrenhal wiped out House Hoare from the map of Westeros. As the halls and towers of Harrenhal were ruined, the castle was correctly depicted to be in a dilapidated state in Game of Thrones as well as House of the Dragon.

Aftermath

Aegon's burning of Harrenhal served as a warning for those set on opposing the Targaryen assertions. The siblings came to Westeros to unite the continent into a single political entity using Fire and Blood, and they lived every bit up to their ruthless reputation. The Ironborn who exploited the Rivermen fled, and those who survived the assault submitted to Aegon. Since Aegon still needed a new Ironborn family to be placed in charge, he allowed the people to choose a leader among themselves. Aegon would be the premier, and the newly chosen House Greyjoy of Pyke ruled the Iron Islands for the next three centuries.

Moreover, Aegon also undertook the reorganization of the Riverlands and it became a separate administrative area. This brought the total constituent regions of the Seven Kingdoms to eight. He also declared the Crownlands as a separate administrative area and the ninth distinct region of Westeros. Earlier, House Hoare ruled the Iron Islands and the Riverlands but now, House Tully (Game of Thrones' Catelyn Stark's family) was granted paramountcy over the Riverlands, and Edmyn was made first Lord Paramount of the Trident. The news of Harrenhal drove the last Storm King - Argilac the Arrogant to face the Targaryens in the field. Aegon's commander, Orys Baratheon, and sister-wife, Rhaenys defeated Argilac in the Battle of the Last Storm. Orys took Storm's End, married Argilac's heir, Argella, and established House Baratheon of Westeros.

Harrenhal In Game Of Thrones

Tywin Lannister and Arya Stark talk about Aegon's burning of Harrenhal in Game of Thrones season 2, episode 7 "A Man Without Honor." Arya who disguised herself as a commoner and served as Tywin's cupbearer at Harrenhal says she knows about Aegon and his sister-wives. Tywin offers Arya his food, and she munches on mutton, he talks about leaving a legacy for his children:

Harren the Black thought this castle would be his legacy. Greatest fortress ever built. Tallest towers, the strongest walls. The Great Hall had thirty-five hearths.

Game of Thrones' "A Man Without Honor," goes into Aegon's burning of Harrenhal. The castle was immune to land attacks, and Harren did not foresee the wrath of the dragon. Aegon had the castle blasted to ruins and by so doing, he changed the rules of combat in Westeros. Tywin concludes by saying Aegon's modus operandi is the reason why he is still remembered three hundred years after his death.

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