Banking is a vital part of the game. Both items and money are localized, they do not magically transfer from one back to another.
Banks can be owned by players or by NPCs. When a player puts their money in an NPC bank, a low penalty is applied. For example, depending on the bank they might lose 1% of the money they put in the bank per week. This creates higher possibilities for player owned and run banks. Players are able to offer much higher interest rates because of high minimum investment opportunities for them.
For example, a ruler of a city might need 10,000 gold to quickly fix his city. A player who owned their own bank would have the capital to be able to lend that much. They could charge a 15% interest rate, and then be giving their investors a 10% interest rate. All financial agreements in the game are finalized on in game documents and signed by both parties.
Players and guilds have a “Reliability Rating” for banking. To prevent massive losses to players, anyone who is a banker must buy insurance. The insurance policy might cost 10,000 gold and cover 10,000 gold in losses. It covers 50% of investment. So a player invests 1000 gold in that bank, 500g of the bank’s insurance credit is gone, and if the player’s money is stolen he gets paid 500g from the insurance policy. As a player or guild holds on to money with no incidents of stealing or security breaches; their Reliability Rating rises, and with it the insurance coverage.
All of this is handled by the Justicars, an NPC-run organization that doesn’t seem to be of any of the races in the game. They are independent and wander throughout the lands. They are on their own faction and can be attacked, but they are heavily defended. They have a small island off to the south, where most of the actual money is kept. This island is one of the most heavily defended in the game. They escort new money to and from ships headed for the island via a caravan with dozens of armed guards.
Instead of carrying large sums of money, players can carry "letters of rights" that allow them to withdraw large amounts of money from other banks. Additionally, banks that are owned by the same group communicate among themselves how much the player has stored with them.