• Originally built by Harren the black who was the last iron king to rule over both the riverlands and iron islands.
• The Ironborn invaded the riverlands and conquered its lands, forcing its people into slavery and forcing them to build Harrenhal. So Harrenhal wasn’t initially controlled by Riverrun it was the other way round.
• He built it as a monument to himself and drained the riverlands of finances and resources as it took 40years to build.
• It is rumoured that Harren mixed the blood of slaves into the mortar used for the stonework in order to prevent any lord from permanently holding it.
• Harrenhal initially fell to Aegon the conqueror
• After conquering Harrenhal Aegon passed control of it to his loyal banner of House Qoherys.
• Following their demise, the houses Towers, then Harroway, then Strong, and then Lothston all had stewardship of Harrenhal.
• During the reign of Aegon the I’s sons, a rebel naming himself Harren the Red, who claimed to be the bastard of Harren the Black led a revolt and took over Harrenhal however his revolt was destroyed.
• The only significant conflict that Harrenhal hasn’t been involved in was Robert Baratheon’s rebellion. It is believed however that Harrenhal was trigger for the rebellion, as it was there that Rhaegar Targaryen named Lyanna Stark the Queen of Beauty and Love. After this Rhaegar kidnapped Lyanna and this led to the beginning of the rebellion.
• During the war of the fiver kings, House Whent controlled Harrenhal. However the crown stripped them of the castle and handed it to house Slynt due to his betrayal of Ned Stark.
• Interestingly Janos Slynt never took control of the castle and the war still raged on. He was then sent to take the Black at the wall thereby stripping him of the castle.
• Tywin Lannister's army then took control of the castle as an operating base against the Young Wolf, however he abandons it as he’s ordered back to Kings landing to help defend the city.
• Robb Stark takes over the empty castle and leaves Roose Bolton as the castellan. Bolton later leaves to attend the wedding of Edmure Tully and Roslin Frey and leaves Locke in charge.
• Interestingly the Young Wolf’s is related to the Whents as his maternal grandmother was from this House, therefore he has an official claim to the castle.
• House Whent were loyal bannermen to Lord Tully of Riverrun thereby enforcing Robb’s claim.
Previous castellans of Harrenhal :
As it is a wide held belief in Westeros that Harrenhal is cursed, Roose Bolton is the only castellan so far not to have befallen some tragedy.
Harrenhal as noted by user568458 is currently in the hands of Littlefinger though he doesn't currently occupy it physically as he's based in the Vale.