Gold ingots are used by the King, the Hand of the King or the Queen to purchase special bonuses in the Treasury that give unique effects or rewards to either one player or to all players within the kingdom. The full list of bonuses that are bought with gold ingots can be seen in the Treasury.
Gold ingots are produced by your city Capitol. The higher the level of the Capitol, the more ingots you'll be able to produce in an hour. Level up your city Capitol in order to get more ingots. You can also attack enemy players and steal their gold ingots in case of a successful attack. Finally, you can score in the top-100 of the Kingdom Clash to obtain 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 your Capitol and then hit "Taxes", then select the amount of ingots you'd like to send and hit "Pay Taxes".