"A Lannisters always pay his debts," a popular phrase by the Lannisters who are known for making bribery and making deals to maintain their power or get out of a sticky situation. One of the most resilient families in Westeros, the Lannisters maybe powerful but there is only one that can bring them down in a snap of a finger.
Best known for their massive wealth which is currently declining, Tywin Lannister will do anything within his power to keep the Iron Throne even if that means marrying the fragile Tommen to Joffrey's widow. But the show's season 4, episode 5 features how serious the threat is to their reign if they can't afford to pay up their debts in the Iron Bank of Braavos.
The bank is the one who funds their costly war against Stannis and the Starks, and without the money the Lannisters will lose their grip on the crown. But this season reveals that the Seven Kingdom's most influential family is now broke and they need the Tyrells to keep them afloat.
"The crown owes The Iron Bank of Braavos a tremendous amount of money," explains Tywin. "That's what the Iron Bank is, a temple. We all live in its shadow, and almost none of us know it. You can't run from them. You can't cheat them. You can't sway them with excuses. If you owe them money, and you don't want to crumble yourself, you pay it back."
With so many budget deficits, Cersie is now cozying with Margaery Tyrell to make her marry Tommen who is now next to the throne. "Tyrells are our only true rivals in terms of resources, and we need them on our side. Vesting the Tyrells in the crown will help a great deal," adds Tywin.
So the question now is how did the Lannisters end up owing the bank with so much debt? "Our last working mine dried up three years ago," Tywin confesses to Cersie. Thanks to Peter "littlefinger" Baelish the Lannisters debt is sky rocketing and it's obvious why he is the man behind the boy king's death.
If the family will fail to pay up the bank, the Iron Bank of Braavos will fund their enemies or get their gold in one way or another. So to make sure they will stay in power, Tywin needed the Tyrells to help them even if that means that Cersie have to marry again.