There are three types of Currency in Fortress: Destroyer; Gold, Alloy, and Circuits. Each of the Currencies can be converted to any other, and they are all used in the creation of certain items (towers, modules, etc.).
“Also called currencies, resources are required to improve your fleet. Gold, alloy, and circuits...Resources
“Gold is a Currency found in Fortress: Destroyer. Gold is used in the creation of every item in...Gold
“Alloy is a currency in Fortress: Destroyer. Alloy is used in the creation of towers and hulls...Alloy
“Circuits are a currency in Fortress: Destroyer. Circuits are the most valuable currency in the...Circuits