The Bank is like a trading outpost, in a sense. The Emperor sponsors the bank, and is willing to let you exchange the resources you have for different resources. You can trade 2 of platinum, ore, or lumber and in return receive 1 of platinum, ore, or lumber. Because the Emperor values gems more highly, he will not let you trade for them, however, he will offer you 2 platinum, ore, or lumber for 1 of your gems. The emperor will also not trade for food, but you can buy food from him for 4 platinum, lumber or ore, or a single gem. Resource management is key to playing the game, and if done perfectly, the bank should be used very rarely. Because the rates are clearly in the bank's favor, you should rely on careful resource management rather than swapping resources at the bank.