There are two types of currency in Mighty Battles, these are bucks and gold. Bucks can be obtained free from opening crates or watching videos and you always have the option of purchasing more bucks using the gold you have earned. Bucks can be spent on purchasing the units that become available to buy for a limited time and to upgrade units currently in your squad.
Gold can be used to purchase premium crates or bucks. Gold is awarded when you level up, earn Achievements, make in-app purchases, complete Tapjoy 'Free Gold!' offers or watch 'Free Gold!' videos.
The Tapjoy 'Free Gold!' offers and 'Free Gold' video features can be found by:
1) Taping the '+' or gold icon on the top right of the screen.
2) Under the 'Gold' menu, tap the Tapjoy 'Free Gold' or 'Watch Video for Free Gold'.
3) Watch a full video or complete offers to earn free gold.