Gold coins, or just gold, is an in-game currency. It can be spent on various improvements, such as increasing the level of traps or the number of Lock Picks.
Gold is also needed to improve the throne and skip the dungeons that you do not want to break into.
There are three ways to get coins:

1. Mining

In the center of your dungeon is the gold mine that constantly supplies you with coins. Upgrade your Gold Mine Capacity and Gold Mine Speed in the upgrade menu to increase your income. To access the upgrade menu, tap on the castle icon in your dungeon or simply swap downwards.

2. Collecting skulls

When somebody dies while attacking your dungeon, they will leave a skull where they died. Simply tap on the sculls to collect them and get extra gold.
Upgrade your Skull Gold Bonus to get more coins for collecting skulls. This is especially effective when you have a good defense.

3. Selling gems
If you don't have enough gold, you can sell one of your gems. To do it, tap on it and drag it to the lower right corner of the screen. The confirmation window will appear. Tap "Yes". 
Please note that this action is irreversible. You will lose rating points the sold gem had. Gems with the lowest rating are sold automatically when your inventory is full (excluding Unique Gems). 
If you don't have enough gold to upgrade something, you can spend blue orbs instead.