The Riverlands is one of the nine major regions of Westeros, though it was not one of the original Seven Kingdoms that existed prior to Aegon’s Conquest. About 100 years before Aegon’s landing, the Riverlands had been conquered by House Hoare of the Iron Islands, and as such, was part of the Kingdom of the Iron Islands. In addition to the Crownlands, the Riverlands is the only other major region of Westeros today that did not exist as one of the original Seven Kingdoms. That said, the Riverlands is an extremely significant region of Westeros with a rich history that dates back as far as all the other regions of Westeros. At the start of the GoT series, House Tully is overlord house of the Riverlands, ruling from the seat of Riverrun.
More so than any other region, the Riverlands are centrally located, sandwiched between the other regions of Westeros. Just south of the North, east of the Iron Islands, west of the Vale and north of the Reach and the Crownlands. Because of its central location and lack of natural barriers such as mountains or oceans, the Riverrlands have always been a populous region of much activity. During times of peace, the Riverlands are a center of trade, with their namesake rivers providing not only commerce for the fisherfolk, but also trade routes between the regions of Westeros. During times of conflict and war, the Riverlands have often served as battlegrounds, with opposing armies commonly converging in some area of the Riverlands.
The people of the Riverlands share many similar ideals as those of the North; they are a humble people that value ideals such as family, honor and loyalty. Because the Riverlands are so large and home to so many smaller houses, the Riverlands can command a large army during times of war. Like most of Westeros, the Riverlands were first populated by the First Men, who later lost control of their lands to the Andals during the Andal Invasion. After ruling the Riverlands for thousand of years, the Andal riverkings they were eventually conquered by the stormkings of the Iron Islands. About 100 years prior to Aegon’s Conquest, the Riverlands had fallen under control of the Kingdom of the Iron Islands, ruled by House Hoare. 40 years prior to Aegon’s arrival, King Harren Hoare, otherwise known as Harren the Black, ordered for Harrenhal to be built; it would be the greatest castle ever built and would solidify his kingdom stretching from the Iron Islands to the Riverlands. Ironically, after taking 40 years to build, on the very day it was completed, Harrenhal was destroyed by Aegon and his dragon as they burned it down, killing King Haren and wiping out his entire bloodline. As result, the Riverlands were no longer ruled by the Iron Islands, and were free to choose an overlord house, so long as they pledged fealty to Aegon and the Iron Throne. House Tully was selected as overlord house and the Riverlands were made one of the great regions of Westeros
Riverrun is an ancient castle and seat of House Tully. From this castle, the Tullys have ruled over the Riverlands for hundreds of years since Aegon’s Conquest. During times of war, the gates of Riverrun can be opened, flooding the moat around the castle and turning it into a manmade island.