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.
Gold Ingots are produced by your City's Capitol. The higher the level of the Capitol, the more Ingots you'll be able to produce in an hour. Level up your City's Capitol in order to get more Ingots. You can also attack enemy players and steal their Gold Ingots if your attack is successful. Finally, if you reach the top 100 in the Clash of Kingdoms, you'll get at least 300K Gold Ingots.
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.