One of the largest bottlenecks in the game is gold. Nobody ever has enough. The hunt can be exhausting and even when you've accumulated a large pile it's gone in minutes. Luckily there are a few sources of reliable gold at your disposal.
Levy:
Levy everyday, with every gem you can spare, up to the point where it starts to cost 160 gems per levy. This will get you millions a day that you'll spend 10 seconds later leaving you wishing for more. Definitely make sure to max your Coinage tech in the Advancement Tree to maximize the benefit of Levys.
Knights:
The knights that spawn during event times are a decent source of gold even though they have a chance to not drop any gold when you hit them. They should be your fairly high on your priority list to find and hit during events and they can bring in millions a day.
Vikings:
The beginning of the week brings Vikings and their gold chests. Catching as many rallies for them is a huge priority when the event is active. 10s of millions of gold is easy to do in a single day using Vikings. Here is the haul from just 45 Medium Vikings Chests:
Goblins:
Outside event times goblins are the only reliable source of gold besides levying. Port to about 150km from the edge of the map and you'll find enough to keep 5 marches going at a time. I've easily collected as much as 5-10 million gold an hour hunting goblins furiously.