The Capitol is the most important building in your City. It determines the City's level, the maximum base number of times your clanmates can help you, and the level of Mines that you can build on the Kingdom Map. In addition, the Capitol produces Gold Ingots.
You can access the Taxes tab to view your City's general statistics using the Capitol interface.
Gold Ingots can be used by the King, the Hand of the King, or the Queen to purchase special bonuses in the Treasury that grant unique effects or rewards either to a single player or to all players within a Kingdom. The full list of bonuses that can be purchased using Gold Ingots can be viewed in the Treasury.
Once you have at least one Ingot available, you can send it to your Kingdom's Treasury. To do so, click on your Capitol and then select Taxes. Next, select the number of Ingots you'd like to send and hit Pay Taxes.