Capitol is the most important building in your city. It defines the city’s level, the maximum base clan helps you can get from allies, and the level of mines you can build on the Kingdom map. Apart from that, Capitol produces gold ingots.
You can access the taxes tab and view your city's general statistics directly using the Capitol interface.
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 one player or all players within the kingdom. The complete list of bonuses that are bought with gold ingots can be seen in the Treasury.
Once you have at least one ingot available, you can send it to your Kingdom's Treasury. To do so, click your Capitol, hit "Taxes," select the number of ingots you'd like to send, and hit "Pay Taxes."