"The Lannisters always pay their debts" is the favored catchphrase of what's currently the dominant House and family in Westeros. However, the dirty secret of the Lannisters - different from the other dirty secret between Queen Cersei (Lena Headey) and her twin brother Jaime Lannister (Nikolaj Coster-Waldau), which is more or less common knowledge these days - is that the Lannisters are deeply in debt. For centuries, Casterly Rock and the vast gold deposits in its mines made the Lannisters the wealthiest family in Westeros. The family's deceased patriarch Tywin (Charles Dance) used that wealth to acquire power and influence, launching a series of events that have resulted in three Lannisters seated upon the Iron Throne, including Cersei, its reigning occupant. However, all of this grasping for ever-more-power came at a literal financial cost: the gold mines of Casterly Rock ran dry years ago and the family coffers are depleted. In short, until this week's episode of Game of Thrones, the Lannisters were broke.
In a macro sense, the Lannisters' debts are also the Iron Throne's debts, and there is no entity the Seven Kingdoms owe more money to than the Iron Bank of Braavos. Located across the Narrow Sea in the Free City of Braavos, the Iron Bank is the most dominant and powerful financial institution in Game of Thrones. Its influence spreads across Westeros, Essos, and the unseen reaches of the known world. What's more, the Iron Bank has a catchphrase of its own, one that is rightly feared by all indebted to it: The Iron Bank will have its due.
In this week's episode "The Queen's Justice," a rare instance of the Iron Bank coming to collect a debt occurs. The Iron Bank's primary representative Tycho Nestoris (Mark Gatiss) visits Cersei at King's Landing to discuss the Crown's overdue accounts. The financial peril facing the Lannisters manifests across the entire Seven Kingdoms they are trying to secure their dominion over. To understand how the Iron Throne came to owe untold millions of Gold Dragons to the Iron Bank, we have to go back to uncover the gross malfeasance of Cersei's predecessors.
The Iron Bank and Westeros
The legacy of King Robert Baratheon (Mark Addy) is as a drunken, whoring misanthrope who had no interest in ruling the Seven Kingdoms. He left the actual hard work of running Westeros to his Small Council. Robert's Master of Coin, Lord Petyr Baelish (Aiden Gillen) paid for the Crown's corpulence by borrowing heavily from Tywin Lannister's fortune. Tywin was more than happy to financially support the Iron Thone, and thus the Seven Kingdoms, since it consolidated his personal power, his grandson Joffrey (Jack Gleeson) was heir to the throne, and his daughter Cersei was the Queen. However, Littlefinger was also secretly borrowing money from the Iron Bank, to the point where the Crown was 6 million Gold Dragons in debt to the Lannisters and to the Bank by the time Eddard Stark (Sean Bean) briefly became Hand of the King. Tyrion Lannister (Peter Dinklage) discovered the Seven Kingdoms' financial peril when he assumed the duties of Master of Coin in season 3.
The War of the Five Kings further increased the massive debt the Seven Kingdoms owed to the Iron Bank, worsened by how deeply intertwined the Lannisters' finances were to the Crown's. This was further exacerbated by the fact that Tywin, who became Hand of the King at the end of season 2, was guarding the secret that Casterly Rock's gold mines depleted 3 years prior, and he himself was borrowing money from the Iron Bank to fund the Seven Kingdoms. When Joffrey married Margaery Tyrell (Natalie Dormer) and made her Queen, Tywin intended the union between the Houses of Lannister and Tyrell, the second wealthiest House in Westeros, to provide some financial relief. (It's possible Tywin was plotting to eventually gain control of House Tyrell's wealth and resources to supplement his own, which Cersei would eventually accomplish.)
By the time Tommen Baratheon (Dean Charles-Chapman) became King, the Crown was even further in debt to the Iron Bank - said to be "a tremendous amount of money." The Iron Bank called upon the Iron Throne to pay back one-tenth of the money that was owed. The new Master of Coin, Tommen's father-in-law, Mace Tyrell (Roger Ashton-Griffiths), noted that the Crown only had half of the one-tenth the Iron Bank demanded. Mace was dispatched to Braavos to meet with the Iron Bank and managed to negotiate new terms, but this only delayed full payment of the ever-increasing debt.
A different would-be king of Westeros visited the Iron Bank in season 4. Stannis Baratheon (Stephen Dillane) and his Hand Davos Seaworth (Liam Cunningham) called upon the Iron Bank to fund their failing campaign in Westeros. At first rebuffed, Davos convinced the Iron Bank that the Lannisters were unloved by the people, untrustworthy, and incompetent compared to Stannis' steady hand and true claim to the Iron Throne. The Iron Bank agreed to an arrangement for a provisional loan and Stannis used the influx of funds to finance his campaign to help the Night's Watch defeat the White Walkers in the Battle of Castle Black. When Stannis died in his foolhardy attempt to siege and take Winterfell from Ramsay Bolton (Iwan Rheon), the Iron Bank's investment went up in smoke.