Gold obtained as hourly tribute from the people who live in the islands you free from BlackGuard slavery. You can use an upgraded Radar to find these islands, and you can also have to dip into your gold reserves to pay to reveal areas of certain interest. Remember that the BlackGuard forces can reclaim islands you have freed, and they may build stronger encampments there, so it is important to monitor activity at all times.
The Headquarters, Vault, and Sawmill are the most important buildings when you first start You lose the round if the Headquarters building is destroyed, since nearly everything else you own is automatically destroyed and any resources you haven't placed in the Vault (which can store 25% of them). As for the Sawmill, it is needed to produce wood, and most upgrades you might make require wood. Therefore, make sure that you keep your Headquarters and Vault in great shape, upgraded as possible so that they will improve your performance in all other areas.
Since wood is vital for most upgrades, especially early in the game, make Sawmill upgrades a priority each time they become available by updating your Headquarters. You can find trees on the map and turn them into wood by spending gold, but those don't reappear so you should rely on them only in tough situations. A recommended strategy is to find wood during conquest, or you can raid other player bases for resources. If they haven't placed their wood in a vault, you can sometimes score some extra wood. In the late stages you may also find additional bases that increase the level of production possible.