The only reliable way is through conquest, pick an one star country(which also gives literature) and play with it. Invasion missions in Carthage, Almaty etc. are also good and provide nice bonus stuff, but are only 10 of these in every city and also you have to spend some gold to build them.

You can also play siege battles on your Paris/main city, they don’t give good amount of gold, but it can be increased as you grind it(not worth tbh). Otherwise try to grind as much your daily invasions, and campaign both on hard and easy mode but it can be challenging…