Gold is the main currency in Wartune. It is used to upgrade Buildings, buy Merceneries, buy Farm seeds, exchange for Guild Contribution, etc. As of the current patch, players can have a maximum of 2,147,483,647 Gold.
City Gold Production
Your Town Hall, Gold Refinery, and Academy (via the Gold Production technology) can each provide a constant hourly gold production. You can also levy an amount of gold based on your city gold production five times per day.
The Wilds contains a scattering of gold mines - usually enough for an active player to find two, which is the maximum number you can occupy at a time. Gold mines provide an additional hourly gold production. At least until they expire and you have to find more.
A few thousand gold can be obtained anywhere that a random item can be obtained (such as after each victory while adventuring, or as the reward for completing a duel or campaign).
In the Wilds, you can find other player's cities dotted around. Attacking a city will cause you to fight that player (exactly like an Arena duel). If you win the fight, you will take a percentage of that player's gold and may also gain some Kyanite.
You can loot cities a maximum of 5 times per day. Additional attempts can be added for 395 Balens each.
Quests and Circuit Quests
Most quests will grant you a quantity of gold upon completion. You can also complete circuit quests to gain gold and daru, these can be done at anytime 200 times a week.
Most items in your inventory can be sold for a quantity of gold. This is only worth doing with items that are otherwise useless to you. Blue-bordered or purple-bordered armor or weapons worse than your current set is usually better recycled. White-bordered or green-bordered items are best sold for gold.