Gold
The premium currency in the game is gold and is earned when you are victorious on the battlefield.
Campaign Keys
When you complete ten Campaign levels you will earn a Campaign key which can be used to open up a Campaign chest that contains a 3-star unit.
Free Chest
Periodically check in throughout the day as players are rewarded every few hours with a new chest in the Officer's Club that features one randomly selected new 1 or 2-star unit that players can either add to their army or scrap for parts at the hanger to make an existing unit stronger. If you enable notifications for the game you will know when you can open your next free chest.