Black Gold is of the game's resources. It is an infused metal used to strengthen towers (used to research and upgrade towers). It's worth 20 gold.
It is obtained through
- Battles and Level Taxes on most of Lava Plains, plus the final and bonus scenarios of The Tundra
- Supply Station
- Prison (at random when you make a capture)
- Daily Check (once an month)
- One of the possible rewards you can get from the Daily Login Reward at the Reward Center
- Events
You can also buy it in Marketplace.
It is used to fortify towers from level 21 until level 51 the Engineering Quarter, and to arm soldiers on Tier 4.
There are two areas in the Grasslands that do not have the potential to provide Black Gold, Scorched Waste and Flaming Plains.
It can be Synthesized into Quality Black Gold at the Forge. You need to level up a tower of any kind to level 51 before you're able to do this.
The max stack of Black Gold is 9999.