The Stormlands, formerly known as the Kingdom of the Stormlands, are a constituent region of the Seven Kingdoms in the Game of Thrones universe. They have a fascinating history, in that, the legendary first Storm King - Durran Godsgrief went to war with the sea god and the goddess of wind for their daughter, Elenei. He managed to raise Storm's End, which withstood the storms in Shipbreaker Bay. Aegon's Conquest decimated the line of the Storm Kings and elevated Orys Baratheon as the Lord of the Stormlands and Storm's End.

Ever since the Targaryen Conquest, the Stormlands have been ruled by a line of Baratheon lords, such as Robert's father, Steffon, Robert himself, Renly, Stannis, and the latest, Gendry, as per Game of Thrones canon. Incidentally, when Robert went to war against the Targaryens, Summerhall, which is located in the Stormlands, was a battleground for Targaryen loyalists and rebel forces. Robert battled three loyalist armies in succession at the outbreak of his rebellion.

The Ruins Of Summerhall

Once a summer castle of members of House Targaryen, Summerhall was destroyed in a deadly fire in 259 AC, courtesy of Aegon the Fifth and the loyalist entourages' attempt to hatch petrified dragon eggs. King Aegon V, his Lord Commander Ser Duncan the Tall, and Prince Duncan perished in the fire, leaving behind a few survivors and the ruins of the castle in Game of Thrones lore. Since Summerhall is strategically situated at the borders of the Reach and Dorne, the garrison that went on to secure the region in Robert's Rebellion achieved an upper hand in the war.

Robert's Rebellion And Three Battles At Summerhall

Took Place In 282 AC

Robert Baratheon went to war against the Mad King and crown prince Rhaegar Targaryen in response to his betrothed Lyanna Stark's alleged kidnapping. As seen in Game of Thrones, Lyanna and Rhaegar were in love and ran away secretively but left the realm to bleed. Since Robert was the Lord of the Stormlands, his banners were torn between their oath to House Baratheon and the monarch on the Iron Throne. The Stormlanders who cast aside their liege lord and declared for Aerys II were Lord Fell of Fellwood, Lord Cafferen of Fawnton, and Lord Grandison of Grandview. These three lords were to assemble at Summerhall to march to Robert's seat, Storm's End. Once Robert received news of the loyalists' approach to combat, he set off to Summerhall and defeated all three of them in three separate battles that took place one after the other within a single day. Lord Fell died at the hands of Robert in a single combat. It is important to remember that Robert defeated Houses Fell, Cafferen, and Grandison before they even had the chance to coordinate.

In Game of Thrones season 1, episode 3 "Lord Snow" Mark Addy's Robert I Baratheon recalls an interesting war story from Three Battles at Summerhall to Lord Commander Ser Barristan Selmy. He tells him his first kill was a Tarly boy at Summerhall:

Mine was some Tarly boy at the Battle of Summerhall. My horse took an arrow, so I was on foot, slogging through the mud. He came running at me, this dumb highborn lad, thinking he could end the rebellion with a single swing of his sword. I knocked him down with the hammer. Gods I was strong then. Caved in his breastplate. Probably shattered every rib he had... Now the Tarlys bend the knee like everyone else.

House Tarly of Horn Hill is a powerful noble House in the Reach, formerly sworn to House Tyrell of Highgarden. It has given the realm men such as Samwell Tarly, and Lord Randyll Tarly. The Tarlys and the Tyrells fought for the loyalist side during Robert's Rebellion. The Battle of Summerhall (or, Three Battles at Summerhall) was followed by the indecisive Battle of Ashford, which was fought in the far west within the Reach. The seasoned warrior, Lord Randyll Tarly, led the vanguard of the Reach. He defeated the rebels in the ensuing battle and forced them to retreat.

The Aftermath Of Three Battles At Summerhall

The prisoners of war from Summerhall, namely, Lord Fell's son, Silveraxe, Lord Grandison, and Lord Cafferen were taken to Storm's End. They defected in time, and Lord Cafferen ended up dying at the hands of Randyll Tarly at Ashford. Lord Grandison fought and succumbed to injuries after fighting at Trident. The Stormlands rallied behind Robert after his ​​​​impressive victory at Summerhall. His army was bigger than before, but he hastily went into the west, leaving his younger brother, Stannis, in charge of Storm's End.

Robert couldn't have won the war without Ned Stark, Jon Arryn, and Hoster Tully's help. He acknowledges Ned's contribution while naming him the Hand of the King in Game of Thrones:

You helped me win the Iron Throne, now help me keep the damn thing.