Wood is key. You'll need wood to make most upgrades, particularly early in the game, so you should 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 tight situations. A better 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 a bunch of extra wood. In the late stages, you may also find additional bases that increase the level of production possible.