The best way to earn gold is by collecting it from the houses/businesses. Each house/business gives you a different amount of gold at a different rate. For example, if you see the picture below, you will see that the Fairy House gives 75 gold every 2 hours:
When a house's/business' gold is ready to be collected, a money bag will appear in front of it.
You can interrupt a business' contract at any time by clicking on the building to collect objects before the time is up, but be careful as if you click on it too much, your workers will go on strike and you will have to collect materials to resolve the strike.
If you want to earn even more gold from your houses/businesses, you can place decorations near them to boost their gold output (See Article 'Boost Buildings').
You can also earn gold by whacking or petting creatures, completing goals, leveling up and by visiting your 1st ten neighbors and completing 5 actions within their Kingdom (it is the 1st 5 neighbors if you are Level 1-14 and the 1st 8 neighbors if you are Level 15 or 16).