Defeat the spider Boss in Everlight Temple. Use the elevator to exit. After you have the Netherworld Gate to this zone unlocked, return to the temple, and open the secret room at the top of the stairs by using the Overlord switch. You can collect at least 1,000 gold from the treasure chests inside the secret room. Warp to Nordburg Town Gate 3 to reset the temple area. You can also loot the treasure chests next to the Tower Gate. Go back and forth between those two areas to get easy gold.
Enslave both Nordberg and Everlight for large chests of gold that spawn when the area is reset. Teleport between the two locations to collect from both of them and reset the other area for your return. Each set of chests contains at least 1,000 gold. Note: You must completely dominate a town to get 1,000 gold from the chest spawning sites. All 100 citizens must be enslaved. For each 10 citizens not enslaved, 100 gold will be subtracted.
After finishing the assault on Empire City, return there to find no troops, several boxes with the highest grade armor and weapons, and over 2,000 gold per visit.